Dark matter is a type of matter in astronomy
and cosmology hypothesized to account for
effects that appear to be the result of invisible
mass. Dark matter cannot be seen directly
with telescopes; evidently it neither emits
nor absorbs light or other electromagnetic
radiation at any significant level. It is
otherwise hypothesized to simply be matter
that is not reactant to light. Instead, the
existence and properties of dark matter are
inferred from its gravitational effects on
visible matter, radiation, and the large-scale
structure of the universe. According to the
Planck mission team, and based on the standard
model of cosmology, the total mass–energy
of the known universe contains 4.9% ordinary
matter, 26.8% dark matter and 68.3% dark energy.
Thus, dark matter is estimated to constitute
84.5% of the total matter in the universe,
while dark energy plus dark matter constitute
95.1% of the total content of the universe.
Astrophysicists hypothesized dark matter because
of discrepancies between the mass of large
astronomical objects determined from their
gravitational effects and the mass calculated
from the "luminous matter" they contain: stars,
gas, and dust. It was first postulated by
Jan Oort in 1932 to account for the orbital
velocities of stars in the Milky Way and by
Fritz Zwicky in 1933 to account for evidence
of "missing mass" in the orbital velocities
of galaxies in clusters. Subsequently, many
other observations have indicated the presence
of dark matter in the universe, including
the rotational speeds of galaxies by Vera
Rubin in the 1960s–1970s, gravitational
lensing of background objects by galaxy clusters
such as the Bullet Cluster, the temperature
distribution of hot gas in galaxies and clusters
of galaxies, and more recently the pattern
of anisotropies in the cosmic microwave background.
According to consensus among cosmologists,
dark matter is composed primarily of a not
yet characterized type of subatomic particle.
The search for this particle, by a variety
of means, is one of the major efforts in particle
physics today.
Although the existence of dark matter is generally
accepted by the mainstream scientific community,
some alternative theories of gravity have
been proposed, such as MOND and TeVeS, which
try to account for the anomalous observations
without requiring additional matter.
Overview
Dark matter's existence is inferred from gravitational
effects on visible matter and gravitational
lensing of background radiation, and was originally
hypothesized to account for discrepancies
between calculations of the mass of galaxies,
clusters of galaxies and the entire universe
made through dynamical and general relativistic
means, and calculations based on the mass
of the visible "luminous" matter these objects
contain: stars and the gas and dust of the
interstellar and intergalactic medium.
The most widely accepted explanation for these
phenomena is that dark matter exists and that
it is most probably composed of weakly interacting
massive particles that interact only through
gravity and the weak force. Alternative explanations
have been proposed, and there is not yet sufficient
experimental evidence to determine whether
any of them are correct. Many experiments
to detect proposed dark matter particles through
non-gravitational means are under way.
According to observations of structures larger
than star systems, as well as Big Bang cosmology
interpreted under the Friedmann equations
and the Friedmann–Lemaître–Robertson–Walker
metric, dark matter accounts for 26.8% of
the mass-energy content of the observable
universe. In comparison, ordinary matter accounts
for only 4.9% of the mass-energy content of
the observable universe, with the remainder
being attributable to dark energy. From these
figures, matter accounts for 31.7% of the
mass-energy content of the universe, and 84.5%
of the matter is dark matter.
Dark matter plays a central role in state-of-the-art
modeling of cosmic structure formation and
Galaxy formation and evolution and has measurable
effects on the anisotropies observed in the
cosmic microwave background. All these lines
of evidence suggest that galaxies, clusters
of galaxies, and the universe as a whole contain
far more matter than that which interacts
with electromagnetic radiation.
Important as dark matter is thought to be
in the cosmos, direct evidence of its existence
and a concrete understanding of its nature
have remained elusive. Though the theory of
dark matter remains the most widely accepted
theory to explain the anomalies in observed
galactic rotation, some alternative theoretical
approaches have been developed which broadly
fall into the categories of modified gravitational
laws and quantum gravitational laws.
Baryonic and nonbaryonic dark matter
There are three separate lines of evidence
that the majority of dark matter is not made
of baryons:
The theory of Big Bang nucleosynthesis, which
very accurately predicts the observed abundance
of the chemical elements, predicts that baryonic
matter accounts for around 4–5 percent of
the of the Universe. In contrast, evidence
from large-scale structure and other observations
indicates that the total matter density is
about 30% of the critical density.
Large astronomical searches for gravitational
microlensing, including the MACHO, EROS and
OGLE projects, have shown that only a small
fraction of the dark matter in the Milky Way
can be hiding in dark compact objects; the
excluded range covers objects above half the
Earth's mass up to 30 solar masses, excluding
nearly all the plausible candidates.
Detailed analysis of the small irregularities
in the cosmic microwave background observed
by WMAP and Planck shows that around five-sixths
of the total matter is in a form which does
not interact significantly with ordinary matter
or photons.
A small proportion of dark matter may be baryonic
dark matter: astronomical bodies, such as
massive compact halo objects, that are composed
of ordinary matter but which emit little or
no electromagnetic radiation. Study of nucleosynthesis
in the Big Bang produces an upper bound on
the amount of baryonic matter in the universe,
which indicates that the vast majority of
dark matter in the universe cannot be baryons,
and thus does not form atoms. It also cannot
interact with ordinary matter via electromagnetic
forces; in particular, dark matter particles
do not carry any electric charge.
Candidates for nonbaryonic dark matter are
hypothetical particles such as axions, or
supersymmetric particles; neutrinos can only
form a small fraction of the dark matter,
due to limits from large-scale structure and
high-redshift galaxies. Unlike baryonic dark
matter, nonbaryonic dark matter does not contribute
to the formation of the elements in the early
universe and so its presence is revealed only
via its gravitational attraction. In addition,
if the particles of which it is composed are
supersymmetric, they can undergo annihilation
interactions with themselves, possibly resulting
in observable by-products such as gamma rays
and neutrinos.
Nonbaryonic dark matter is classified in terms
of the mass of the particle(s) that is assumed
to make it up, and/or the typical velocity
dispersion of those particles. There are three
prominent hypotheses on nonbaryonic dark matter,
called cold dark matter, warm dark matter,
and hot dark matter; some combination of these
is also possible. The most widely discussed
models for nonbaryonic dark matter are based
on the cold dark matter hypothesis, and the
corresponding particle is most commonly assumed
to be a weakly interacting massive particle.
Hot dark matter may include neutrinos, but
observations imply that only a small fraction
of dark matter can be hot. Cold dark matter
leads to a "bottom-up" formation of structure
in the universe while hot dark matter would
result in a "top-down" formation scenario;
since the late 1990s, the latter has been
ruled out by observations of high-redshift
galaxies such as the Hubble Ultra-Deep Field.
Observational evidence
The first person to interpret evidence and
infer the presence of dark matter was Dutch
astronomer Jan Oort, a pioneer in radio astronomy,
in 1932. Oort was studying stellar motions
in the local galactic neighbourhood and found
that the mass in the galactic plane must be
more than the material that could be seen,
but this measurement was later determined
to be essentially erroneous. In 1933 the Swiss
astrophysicist Fritz Zwicky, who studied clusters
of galaxies while working at the California
Institute of Technology, made a similar inference.
Zwicky applied the virial theorem to the Coma
cluster of galaxies and obtained evidence
of unseen mass. Zwicky estimated the cluster's
total mass based on the motions of galaxies
near its edge and compared that estimate to
one based on the number of galaxies and total
brightness of the cluster. He found that there
was about 400 times more estimated mass than
was visually observable. The gravity of the
visible galaxies in the cluster would be far
too small for such fast orbits, so something
extra was required. This is known as the "missing
mass problem". Based on these conclusions,
Zwicky inferred that there must be some non-visible
form of matter which would provide enough
of the mass and gravity to hold the cluster
together.
Much of the evidence for dark matter comes
from the study of the motions of galaxies.
Many of these appear to be fairly uniform,
so by the virial theorem, the total kinetic
energy should be half the total gravitational
binding energy of the galaxies. Observationally,
however, the total kinetic energy is found
to be much greater: in particular, assuming
the gravitational mass is due to only the
visible matter of the galaxy, stars far from
the center of galaxies have much higher velocities
than predicted by the virial theorem. Galactic
rotation curves, which illustrate the velocity
of rotation versus the distance from the galactic
center, cannot be explained by only the visible
matter. Assuming that the visible material
makes up only a small part of the cluster
is the most straightforward way of accounting
for this. Galaxies show signs of being composed
largely of a roughly spherically symmetric,
centrally concentrated halo of dark matter
with the visible matter concentrated in a
disc at the center. Low surface brightness
dwarf galaxies are important sources of information
for studying dark matter, as they have an
uncommonly low ratio of visible matter to
dark matter, and have few bright stars at
the center which would otherwise impair observations
of the rotation curve of outlying stars.
Gravitational lensing observations of galaxy
clusters allow direct estimates of the gravitational
mass based on its effect on light from background
galaxies, since large collections of matter
will gravitationally deflect light. In clusters
such as Abell 1689, lensing observations confirm
the presence of considerably more mass than
is indicated by the clusters' light alone.
In the Bullet Cluster, lensing observations
show that much of the lensing mass is separated
from the X-ray-emitting baryonic mass. In
July 2012, lensing observations were used
to identify a "filament" of dark matter between
two clusters of galaxies, as cosmological
simulations have predicted.
Galaxy rotation curves
After Zwicky's initial observations, the first
indication that the mass to light ratio was
anything other than unity came from measurements
made by Horace W. Babcock. In 1939, Babcock
reported in his PhD thesis measurements of
the rotation curve for the Andromeda nebula
which suggested that the mass-to-luminosity
ratio increases radially. He, however, attributed
it to either absorption of light within the
galaxy or modified dynamics in the outer portions
of the spiral and not to any form of missing
matter. In the late 1960s and early 1970s,
Vera Rubin, a young astronomer at the Department
of Terrestrial Magnetism at the Carnegie Institution
of Washington, worked with a new sensitive
spectrograph that could measure the velocity
curve of edge-on spiral galaxies to a greater
degree of accuracy than had ever before been
achieved. Together with fellow staff-member
Kent Ford, Rubin announced at a 1975 meeting
of the American Astronomical Society the discovery
that most stars in spiral galaxies orbit at
roughly the same speed, which implied that
the mass densities of the galaxies were uniform
well beyond the regions containing most of
the stars, a result independently found in
1978. An influential paper presented Rubin's
results in 1980. Rubin's observations and
calculations showed that most galaxies must
contain about six times as much “dark”
mass as can be accounted for by the visible
stars. Eventually other astronomers began
to corroborate her work and it soon became
well-established that most galaxies were dominated
by "dark matter":
Low Surface Brightness galaxies. LSBs are
probably everywhere dark matter-dominated,
with the observed stellar populations making
only a small contribution to rotation curves.
Such a property is extremely important because
it allows one to avoid the difficulties associated
with the deprojection and disentanglement
of the dark and visible contributions to the
rotation curves.
Spiral Galaxies. Rotation curves of both low
and high surface luminosity galaxies appear
to suggest a universal density profile, which
can be expressed as the sum of an exponential
thin stellar disk, and a spherical dark matter
halo with a flat core of radius r0 and density
ρ0 = 4.5 × 10−2(r03 M⊙pc−3.
Elliptical galaxies. Some elliptical galaxies
show evidence for dark matter via strong gravitational
lensing, X-ray evidence reveals the presence
of extended atmospheres of hot gas that fill
the dark haloes of isolated ellipticals and
whose hydrostatic support provides evidence
for dark matter. Other ellipticals have low
velocities in their outskirts and were interpreted
as not having dark matter haloes. However,
simulations of disk-galaxy mergers indicate
that stars were torn by tidal forces from
their original galaxies during the first close
passage and put on outgoing trajectories,
explaining the low velocities even with a
DM halo. More research is needed to clarify
this situation.
Simulated dark matter haloes have significantly
steeper density profiles than are inferred
from observations, which is a problem for
cosmological models with dark matter at the
smallest scale of galaxies as of 2008. This
may only be a problem of resolution: star-forming
regions which might alter the dark matter
distribution via outflows of gas have been
too small to resolve and model simultaneously
with larger dark matter clumps. A recent simulation
of a dwarf galaxy resolving these star-forming
regions reported that strong outflows from
supernovae remove low-angular-momentum gas,
which inhibits the formation of a galactic
bulge and decreases the dark matter density
to less than half of what it would have been
in the central kiloparsec. These simulation
predictions—bulgeless and with shallow central
dark matter profiles—correspond closely
to observations of actual dwarf galaxies.
There are no such discrepancies at the larger
scales of clusters of galaxies and above,
or in the outer regions of haloes of galaxies.
Exceptions to this general picture of dark
matter haloes for galaxies appear to be galaxies
with mass-to-light ratios close to that of
stars. Subsequent to this, numerous observations
have been made that do indicate the presence
of dark matter in various parts of the cosmos,
such as observations of the cosmic microwave
background, of supernovas used as distance
measures, of gravitational lensing at various
scales, and many types of sky survey. Together
with Rubin's findings for spiral galaxies
and Zwicky's work on galaxy clusters, the
observational evidence for dark matter has
been collecting over the decades to the point
that by the 1980s most astrophysicists accepted
its existence. As a unifying concept, dark
matter is one of the dominant features considered
in the analysis of structures on the order
of galactic scale and larger.
Velocity dispersions of galaxies
In astronomy, the velocity dispersion σ,
is the range of velocities about the mean
velocity for a group of objects, such as a
cluster of stars about a galaxy.
Rubin's pioneering work has stood the test
of time. Measurements of velocity curves in
spiral galaxies were soon followed up with
velocity dispersions of elliptical galaxies.
While sometimes appearing with lower mass-to-light
ratios, measurements of ellipticals still
indicate a relatively high dark matter content.
Likewise, measurements of the diffuse interstellar
gas found at the edge of galaxies indicate
not only dark matter distributions that extend
beyond the visible limit of the galaxies,
but also that the galaxies are virialized
up to ten times their visible radii. This
has the effect of pushing up the dark matter
as a fraction of the total amount of gravitating
matter from 50% measured by Rubin to the now
accepted value of nearly 95%.
There are places where dark matter seems to
be a small component or totally absent. Globular
clusters show little evidence that they contain
dark matter, though their orbital interactions
with galaxies do show evidence for galactic
dark matter. For some time, measurements of
the velocity profile of stars seemed to indicate
concentration of dark matter in the disk of
the Milky Way galaxy. It now appears, however,
that the high concentration of baryonic matter
in the disk of the galaxy can account for
this motion. Galaxy mass profiles are thought
to look very different from the light profiles.
The typical model for dark matter galaxies
is a smooth, spherical distribution in virialized
halos. Such would have to be the case to avoid
small-scale dynamical effects. Recent research
reported in January 2006 from the University
of Massachusetts Amherst would explain the
previously mysterious warp in the disk of
the Milky Way by the interaction of the Large
and Small Magellanic Clouds and the predicted
20 fold increase in mass of the Milky Way
taking into account dark matter.
In 2005, astronomers from Cardiff University
claimed to have discovered a galaxy made almost
entirely of dark matter, 50 million light
years away in the Virgo Cluster, which was
named VIRGOHI21. Unusually, VIRGOHI21 does
not appear to contain any visible stars: it
was seen with radio frequency observations
of hydrogen. Based on rotation profiles, the
scientists estimate that this object contains
approximately 1000 times more dark matter
than hydrogen and has a total mass of about
1/10 that of the Milky Way Galaxy we live
in. For comparison, the Milky Way is estimated
to have roughly 10 times as much dark matter
as ordinary matter. Models of the Big Bang
and structure formation have suggested that
such dark galaxies should be very common in
the universe, but none had previously been
detected. If the existence of this dark galaxy
is confirmed, it provides strong evidence
for the theory of galaxy formation and poses
problems for alternative explanations of dark
matter.
There are some galaxies whose velocity profile
indicates an absence of dark matter, such
as NGC 3379.
Galaxy clusters and gravitational lensing
Galaxy clusters are especially important for
dark matter studies since their masses can
be estimated in three independent ways:
From the scatter in radial velocities of the
galaxies within them.
From X-rays emitted by very hot gas within
the clusters. The temperature and density
of the gas can be estimated from the energy
and flux of the X-rays, hence the gas pressure;
assuming pressure and gravity balance, this
enables the mass profile of the cluster to
be derived. Many of the experiments of the
Chandra X-ray Observatory use this technique
to independently determine the mass of clusters.
These observations generally indicate a ratio
of baryonic to total mass approximately 12-15
percent, in reasonable agreement with the
Planck spacecraft cosmic average of 15.5 - 16
percent.
From their gravitational lensing effects on
background objects, usually more distant galaxies.
This is observed as "strong lensing" near
the cluster core, and weak lensing in the
outer parts. Several large Hubble projects
have used this method to measure cluster masses.
Generally these three methods are in reasonable
agreement, that clusters contain much more
matter than the visible galaxies and gas.
A gravitational lens is formed when the light
from a more distant source is "bent" around
a massive object between the source object
and the observer. The process is known as
gravitational lensing.
The galaxy cluster Abell 2029 is composed
of thousands of galaxies enveloped in a cloud
of hot gas, and an amount of dark matter equivalent
to more than 1014 Suns. At the center of this
cluster is an enormous, elliptically shaped
galaxy that is thought to have been formed
from the mergers of many smaller galaxies.
The measured orbital velocities of galaxies
within galactic clusters have been found to
be consistent with dark matter observations.
Another important tool for future dark matter
observations is gravitational lensing. Lensing
relies on the effects of general relativity
to predict masses without relying on dynamics,
and so is a completely independent means of
measuring the dark matter. Strong lensing,
the observed distortion of background galaxies
into arcs when the light passes through a
gravitational lens, has been observed around
a few distant clusters including Abell 1689.
By measuring the distortion geometry, the
mass of the cluster causing the phenomena
can be obtained. In the dozens of cases where
this has been done, the mass-to-light ratios
obtained correspond to the dynamical dark
matter measurements of clusters.
Weak gravitational lensing looks at minute
distortions of galaxies observed in vast galaxy
surveys due to foreground objects through
statistical analyses. By examining the apparent
shear deformation of the adjacent background
galaxies, astrophysicists can characterize
the mean distribution of dark matter by statistical
means and have found mass-to-light ratios
that correspond to dark matter densities predicted
by other large-scale structure measurements.
The correspondence of the two gravitational
lens techniques to other dark matter measurements
has convinced almost all astrophysicists that
dark matter actually exists as a major component
of the universe's composition.
The most direct observational evidence to
date for dark matter is in a system known
as the Bullet Cluster. In most regions of
the universe, dark matter and visible material
are found together, as expected because of
their mutual gravitational attraction. In
the Bullet Cluster, a collision between two
galaxy clusters appears to have caused a separation
of dark matter and baryonic matter. X-ray
observations show that much of the baryonic
matter in the system is concentrated in the
center of the system. Electromagnetic interactions
between passing gas particles caused them
to slow down and settle near the point of
impact. However, weak gravitational lensing
observations of the same system show that
much of the mass resides outside of the central
region of baryonic gas. Because dark matter
does not interact by electromagnetic forces,
it would not have been slowed in the same
way as the X-ray visible gas, so the dark
matter components of the two clusters passed
through each other without slowing down substantially.
This accounts for the separation. Unlike the
galactic rotation curves, this evidence for
dark matter is independent of the details
of Newtonian gravity, so it is claimed to
be direct evidence of the existence of dark
matter. Another galaxy cluster, known as the
Train Wreck Cluster/Abell 520, appears to
have an unusually massive and dark core containing
few of the cluster's galaxies, which presents
problems for standard dark matter models.
This may be explained by the dark core actually
being a long, low-density dark matter filament
along the line of sight, projected onto the
cluster core.
The observed behavior of dark matter in clusters
constrains whether and how much dark matter
scatters off other dark matter particles,
quantified as its self-interaction cross section.
More simply, the question is whether the dark
matter has pressure, and thus can be described
as a perfect fluid. The distribution of mass
in galaxy clusters has been used to argue
both for and against the existence of significant
self-interaction in dark matter. Specifically,
the distribution of dark matter in merging
clusters such as the Bullet Cluster shows
that dark matter scatters off other dark matter
particles only very weakly if at all.
Cosmic microwave background
Angular fluctuations in the cosmic microwave
background spectrum provide evidence for dark
matter. Since the 1964 discovery and confirmation
of the CMB radiation, many measurements of
the CMB have supported and constrained this
theory. The NASA Cosmic Background Explorer
found that the CMB spectrum is a blackbody
spectrum with a temperature of 2.726 K. In
1992, COBE detected fluctuations in the CMB
spectrum, at a level of about one part in
105. During the following decade, CMB anisotropies
were further investigated by a large number
of ground-based and balloon experiments. The
primary goal of these experiments was to measure
the angular scale of the first acoustic peak
of the power spectrum of the anisotropies,
for which COBE did not have sufficient resolution.
In 2000–2001, several experiments, most
notably BOOMERanG found the Universe to be
almost spatially flat by measuring the typical
angular size of the anisotropies. During the
1990s, the first peak was measured with increasing
sensitivity and by 2000 the BOOMERanG experiment
reported that the highest power fluctuations
occur at scales of approximately one degree.
These measurements were able to rule out cosmic
strings as the leading theory of cosmic structure
formation, and suggested cosmic inflation
was the right theory.
A number of ground-based interferometers provided
measurements of the fluctuations with higher
accuracy over the next three years, including
the Very Small Array, the Degree Angular Scale
Interferometer and the Cosmic Background Imager.
DASI made the first detection of the polarization
of the CMB, and the CBI provided the first
E-mode polarization spectrum with compelling
evidence that it is out of phase with the
T-mode spectrum. COBE's successor, the Wilkinson
Microwave Anisotropy Probe has provided the
most detailed measurements of anisotropies
in the CMB as of 2009 with ESA's Planck spacecraft
returning more detailed results in 2012-2014.
WMAP's measurements played the key role in
establishing the current Standard Model of
Cosmology, namely the Lambda-CDM model, a
flat universe dominated by dark energy, supplemented
by dark matter and atoms with density fluctuations
seeded by a Gaussian, adiabatic, nearly scale
invariant process. The basic properties of
this universe are determined by five numbers:
the density of matter, the density of atoms,
the age of the universe, the amplitude of
the initial fluctuations, and their scale
dependence.
A successful Big Bang cosmology theory must
fit with all available astronomical observations,
including the CMB. In cosmology, the CMB is
explained as relic radiation from shortly
after the big bang. The anisotropies in the
CMB are explained as acoustic oscillations
in the photon-baryon plasma whose restoring
force is gravity. Ordinary matter interacts
strongly with radiation whereas, by definition,
dark matter does not. Both affect the oscillations
by their gravity, so the two forms of matter
will have different effects. The typical angular
scales of the oscillations in the CMB, measured
as the power spectrum of the CMB anisotropies,
thus reveal the different effects of baryonic
matter and dark matter. The CMB power spectrum
shows a large first peak and smaller successive
peaks, with three peaks resolved as of 2009.
The first peak tells mostly about the density
of baryonic matter and the third peak mostly
about the density of dark matter, measuring
the density of matter and the density of atoms
in the universe.
Sky surveys and baryon acoustic oscillations
The acoustic oscillations in the early universe
leave their imprint in the visible matter
by Baryon Acoustic Oscillation clustering,
in a way that can be measured with sky surveys
such as the Sloan Digital Sky Survey and the
2dF Galaxy Redshift Survey. These measurements
are consistent with those of the CMB derived
from the WMAP spacecraft and further constrain
the Lambda CDM model and dark matter. Note
that the CMB data and the BAO data measure
the acoustic oscillations at very different
distance scales.
Type Ia supernovae distance measurements
Type Ia supernovae can be used as "standard
candles" to measure extragalactic distances,
and extensive data sets of these supernovae
can be used to constrain cosmological models.
They constrain the dark energy density ΩΛ
= ~0.713 for a flat, Lambda CDM Universe and
the parameter w for a quintessence model.
Once again, the values obtained are roughly
consistent with those derived from the WMAP
observations and further constrain the Lambda
CDM model and dark matter.
Lyman-alpha forest
In astronomical spectroscopy, the Lyman-alpha
forest is the sum of absorption lines arising
from the Lyman-alpha transition of the neutral
hydrogen in the spectra of distant galaxies
and quasars. Observations of the Lyman-alpha
forest can also be used to constrain cosmological
models. These constraints are again in agreement
with those obtained from WMAP data.
Structure formation
Dark matter is crucial to the Big Bang model
of cosmology as a component which corresponds
directly to measurements of the parameters
associated with Friedmann cosmology solutions
to general relativity. In particular, measurements
of the cosmic microwave background anisotropies
correspond to a cosmology where much of the
matter interacts with photons more weakly
than the known forces that couple light interactions
to baryonic matter. Likewise, a significant
amount of non-baryonic, cold matter is necessary
to explain the large-scale structure of the
universe.
Observations suggest that structure formation
in the universe proceeds hierarchically, with
the smallest structures collapsing first and
followed by galaxies and then clusters of
galaxies. As the structures collapse in the
evolving universe, they begin to "light up"
as the baryonic matter heats up through gravitational
contraction and the object approaches hydrostatic
pressure balance. Ordinary baryonic matter
had too high a temperature, and too much pressure
left over from the Big Bang to collapse and
form smaller structures, such as stars, via
the Jeans instability. Dark matter acts as
a compactor of structure. This model not only
corresponds with statistical surveying of
the visible structure in the universe but
also corresponds precisely to the dark matter
predictions of the cosmic microwave background.
This bottom up model of structure formation
requires something like cold dark matter to
succeed. Large computer simulations of billions
of dark matter particles have been used to
confirm that the cold dark matter model of
structure formation is consistent with the
structures observed in the universe through
galaxy surveys, such as the Sloan Digital
Sky Survey and 2dF Galaxy Redshift Survey,
as well as observations of the Lyman-alpha
forest. These studies have been crucial in
constructing the Lambda-CDM model which measures
the cosmological parameters, including the
fraction of the universe made up of baryons
and dark matter.
There are, however, several points of tension
between observation and simulations of structure
formation driven by dark matter. There is
evidence that there are 10 to 100 times fewer
small galaxies than permitted by what the
dark matter theory of galaxy formation predicts.
This is known as the dwarf galaxy problem.
In addition, the simulations predict dark
matter distributions with a very dense cusp
near the centers of galaxies, but the observed
halos are smoother than predicted.
History of the search for its composition
Although dark matter had historically been
inferred by many astronomical observations,
its composition long remained speculative.
Early theories of dark matter concentrated
on hidden heavy normal objects as the possible
candidates for dark matter, collectively known
as massive compact halo objects or MACHOs.
Astronomical surveys for gravitational microlensing,
including the MACHO, EROS and OGLE projects,
along with Hubble telescope searches for ultra-faint
stars, have not found enough of these hidden
MACHOs. Some hard-to-detect baryonic matter,
such as MACHOs and some forms of gas, were
additionally speculated to make a contribution
to the overall dark matter content, but evidence
indicated such would constitute only a small
portion.
Furthermore, data from a number of lines of
other evidence, including galaxy rotation
curves, gravitational lensing, structure formation,
and the fraction of baryons in clusters and
the cluster abundance combined with independent
evidence for the baryon density, indicated
that 85–90% of the mass in the universe
does not interact with the electromagnetic
force. This "nonbaryonic dark matter" is evident
through its gravitational effect. Consequently,
the most commonly held view was that dark
matter is primarily non-baryonic, made of
one or more elementary particles other than
the usual electrons, protons, neutrons, and
known neutrinos. The most commonly proposed
particles then became WIMPs, axions, or sterile
neutrinos, though many other possible candidates
have been proposed.
The dark matter component has much more mass
than the "visible" component of the universe.
Only about 4.6% of the mass-energy of the
Universe is ordinary matter. About 23% is
thought to be composed of dark matter. The
remaining 72% is thought to consist of dark
energy, an even stranger component, distributed
almost uniformly in space and with energy
density non-evolving or slowly evolving with
time. Determining the nature of this dark
matter is one of the most important problems
in modern cosmology and particle physics.
It has been noted that the names "dark matter"
and "dark energy" serve mainly as expressions
of human ignorance, much like the marking
of early maps with "terra incognita".
Dark matter candidates can be approximately
divided into three classes, called cold, warm
and hot dark matter.
These categories do not correspond to an actual
temperature, but instead refer to how fast
the particles were moving, thus how far they
moved due to random motions in the early universe,
before they slowed down due to the expansion
of the Universe – this is an important
distance called the "free streaming length".
Primordial density fluctuations smaller than
this free-streaming length get washed out
as particles move from overdense to underdense
regions, while fluctuations larger than the
free-streaming length are unaffected; therefore
this free-streaming length sets a minimum
scale for structure formation.
Cold dark matter – objects with a free-streaming
length much smaller than a protogalaxy.
Warm dark matter – particles with a free-streaming
length similar to a protogalaxy.
Hot dark matter – particles with a free-streaming
length much larger than a protogalaxy.
Though a fourth category had been considered
early on, called mixed dark matter, it was
quickly eliminated since the discovery of
dark energy.
As an example, Davis et al. wrote in 1985:
Candidate particles can be grouped into three
categories on the basis of their effect on
the fluctuation spectrum. If the dark matter
is composed of abundant light particles which
remain relativistic until shortly before recombination,
then it may be termed "hot". The best candidate
for hot dark matter is a neutrino ... A second
possibility is for the dark matter particles
to interact more weakly than neutrinos, to
be less abundant, and to have a mass of order
1 keV. Such particles are termed "warm dark
matter", because they have lower thermal velocities
than massive neutrinos ... there are at present
few candidate particles which fit this description.
Gravitinos and photinos have been suggested
... Any particles which became nonrelativistic
very early, and so were able to diffuse a
negligible distance, are termed "cold" dark
matter. There are many candidates for CDM
including supersymmetric particles.
The full calculations are quite technical,
but an approximate dividing line is that "warm"
dark matter particles became non-relativistic
when the universe was approximately 1 year
old and 1 millionth of its present size; standard
hot big bang theory implies the universe was
then in the radiation-dominated era, with
a photon temperature 2.7 million K. Standard
physical cosmology gives the particle horizon
size as 2ct in the radiation-dominated era,
thus 2 light-years, and a region of this size
would expand to 2 million light years today.
The actual free-streaming length is roughly
5 times larger than the above length, since
the free-streaming length continues to grow
slowly as particle velocities decrease inversely
with the scale factor after they become non-relativistic;
therefore, in this example the free-streaming
length would correspond to 10 million light-years
or 3 Mpc today, which is around the size containing
on average the mass of a large galaxy.
The above temperature 2.7 million K which
gives a typical photon energy of 250 electron-volts,
so this sets a typical mass scale for "warm"
dark matter: particles much more massive than
this, such as GeV – TeV mass WIMPs, would
become non-relativistic much earlier than
1 year after the Big Bang, thus have a free-streaming
length which is much smaller than a proto-galaxy
and effectively negligible. Conversely, much
lighter particles have a free-streaming length
much larger than a proto-galaxy.
Cold dark matter
Today, cold dark matter is the simplest explanation
for most cosmological observations. "Cold"
dark matter is dark matter composed of constituents
with a free-streaming length much smaller
than the ancestor of a galaxy-scale perturbation.
This is currently the area of greatest interest
for dark matter research, as hot dark matter
does not seem to be viable for galaxy and
galaxy cluster formation, and most particle
candidates become non-relativistic at very
early times, hence are classified as cold.
The composition of the constituents of cold
dark matter is currently unknown. Possibilities
range from large objects like MACHOs or RAMBOs,
to new particles like WIMPs and axions. Possibilities
involving normal baryonic matter include brown
dwarfs, other stellar remnants such as white
dwarfs, or perhaps small, dense chunks of
heavy elements.
Studies of big bang nucleosynthesis and gravitational
lensing have convinced most scientists that
MACHOs of any type cannot be more than a small
fraction of the total dark matter. Black holes
of nearly any mass are ruled out as a primary
dark matter constituent by a variety of searches
and constraints. According to A. Peter: "...the
only really plausible dark-matter candidates
are new particles."
The DAMA/NaI experiment and its successor
DAMA/LIBRA have claimed to directly detect
dark matter particles passing through the
Earth, but many scientists remain skeptical,
as negative results from similar experiments
seem incompatible with the DAMA results.
Many supersymmetric models naturally give
rise to stable dark matter candidates in the
form of the Lightest Supersymmetric Particle.
Separately, heavy sterile neutrinos exist
in non-supersymmetric extensions to the standard
model that explain the small neutrino mass
through the seesaw mechanism.
Warm dark matter
Warm dark matter refers to particles with
a free-streaming length comparable to the
size of a region which subsequently evolved
into a dwarf galaxy. This leads to predictions
which are very similar to cold dark matter
on large scales, including the CMB, galaxy
clustering and large galaxy rotation curves,
but with less small-scale density perturbations.
This reduces the predicted abundance of dwarf
galaxies and may lead to lower density of
dark matter in the central parts of large
galaxies; some researchers consider this may
be a better fit to observations. A challenge
for this model is that there are no very well-motivated
particle physics candidates with the required
mass ~ 300 eV to 3000 eV.
There have been no particles discovered so
far that can be categorized as warm dark matter.
There is a postulated candidate for the warm
dark matter category, which is the sterile
neutrino: a heavier, slower form of neutrino
which does not even interact through the Weak
force unlike regular neutrinos. Interestingly,
some modified gravity theories, such as Scalar-tensor-vector
gravity, also require that a warm dark matter
exist to make their equations work out.
Hot dark matter
Hot dark matter consists of particles that
have a free-streaming length much larger than
a proto-galaxy size.
An example of hot dark matter is already known:
the neutrino. Neutrinos were discovered quite
separately from the search for dark matter,
and long before it seriously began: they were
first postulated in 1930, and first detected
in 1956. Neutrinos have a very small mass:
at least 100,000 times less massive than
an electron. Other than gravity, neutrinos
only interact with normal matter via the weak
force making them very difficult to detect.
This would classify them as Weakly Interacting
Light Particles, or WILPs, as opposed to cold
dark matter's theoretical candidates, the
WIMPs.
There are three different known flavors of
neutrinos, and their masses are slightly different.
The resolution to the solar neutrino problem
demonstrated that these three types of neutrinos
actually change and oscillate from one flavor
to the others and back as they are in-flight.
It's hard to determine an exact upper bound
on the collective average mass of the three
neutrinos. For example, if the average neutrino
mass were chosen to be over 50 eV/c2, just
by the sheer number of them in the universe,
the universe would collapse due to their mass.
So other observations have served to estimate
an upper-bound for the neutrino mass. Using
cosmic microwave background data and other
methods, the current conclusion is that their
average mass probably does not exceed 0.3
eV/c2 Thus, the normal forms of neutrinos
cannot be responsible for the measured dark
matter component from cosmology.
Hot dark matter was popular for a time in
the early 1980s, but it suffers from a severe
problem: since all galaxy-size density fluctuations
get washed out by free-streaming, the first
objects which can form are huge supercluster-size
pancakes, which then were theorised somehow
to fragment into galaxies. Deep-field observations
clearly show that galaxies formed at early
times, with clusters and superclusters forming
later as galaxies clump together, so any model
dominated by hot dark matter is seriously
in conflict with observations.
Mixed dark matter
Mixed dark matter is a now obsolete model,
with a specifically chosen mass ratio of 80%
cold dark matter and 20% hot dark matter content.
Though it is presumable that hot dark matter
coexists with cold dark matter in any case,
there was a very specific reason for choosing
this particular ratio of hot to cold dark
matter in this model. During the early 1990s
it became steadily clear that a Universe with
critical density of cold dark matter did not
fit the COBE and large-scale galaxy clustering
observations; either the 80/20 mixed dark
matter model, or LambdaCDM, were able to reconcile
these. With the discovery of the accelerating
universe from supernovae, and more accurate
measurements of CMB anisotropy and galaxy
clustering, the mixed dark matter model was
essentially ruled out while the concordance
LambdaCDM model remained a good fit.
Detection
If the dark matter within our galaxy is made
up of Weakly Interacting Massive Particles,
then thousands of WIMPs must pass through
every square centimeter of the Earth each
second. There are many experiments currently
running, or planned, aiming to test this hypothesis
by searching for WIMPs. Although WIMPs are
the historically more popular dark matter
candidate for searches, there are experiments
searching for other particle candidates; the
Axion Dark Matter eXperiment is currently
searching for the dark matter axion, a well-motivated
and constrained dark matter source. It is
also possible that dark matter consists of
very heavy hidden sector particles which only
interact with ordinary matter via gravity.
These experiments can be divided into two
classes: direct detection experiments, which
search for the scattering of dark matter particles
off atomic nuclei within a detector; and indirect
detection, which look for the products of
WIMP annihilations.
An alternative approach to the detection of
WIMPs in nature is to produce them in the
laboratory. Experiments with the Large Hadron
Collider may be able to detect WIMPs produced
in collisions of the LHC proton beams. Because
a WIMP has negligible interactions with matter,
it may be detected indirectly as missing energy
and momentum which escape the LHC detectors,
provided all the other collision products
are detected. These experiments could show
that WIMPs can be created, but it would still
require a direct detection experiment to show
that they exist in sufficient numbers in the
galaxy to account for dark matter.
Direct detection experiments
Direct detection experiments typically operate
in deep underground laboratories to reduce
the background from cosmic rays. These include:
the Soudan mine; the SNOLAB underground laboratory
at Sudbury, Ontario; the Gran Sasso National
Laboratory; the Canfranc Underground Laboratory;
the Boulby Underground Laboratory; and the
Deep Underground Science and Engineering Laboratory,
South Dakota.
The majority of present experiments use one
of two detector technologies: cryogenic detectors,
operating at temperatures below 100mK, detect
the heat produced when a particle hits an
atom in a crystal absorber such as germanium.
Noble liquid detectors detect the flash of
scintillation light produced by a particle
collision in liquid xenon or argon. Cryogenic
detector experiments include: CDMS, CRESST,
EDELWEISS, EURECA. Noble liquid experiments
include ZEPLIN, XENON, DEAP, ArDM, WARP, DarkSide
and LUX, the Large Underground Xenon Detector.
Both of these detector techniques are capable
of distinguishing background particles which
scatter off electrons, from dark matter particles
which scatter off nuclei. Other experiments
include SIMPLE and PICASSO.
The DAMA/NaI, DAMA/LIBRA experiments have
detected an annual modulation in the event
rate, which they claim is due to dark matter
particles.. This claim is so far unconfirmed
and difficult to reconcile with the negative
results of other experiments assuming that
the WIMP scenario is correct.
Directional detection of dark matter is a
search strategy based on the motion of the
Solar System around the galactic center.
By using a low pressure TPC, it is possible
to access information on recoiling tracks
and to constrain the WIMP-nucleus kinematics.
WIMPs coming from the direction in which the
Sun is travelling may then be separated from
background noise, which should be isotropic.
Directional dark matter experiments include
DMTPC, DRIFT, Newage and MIMAC.
On 17 December 2009 CDMS researchers reported
two possible WIMP candidate events. They estimate
that the probability that these events are
due to a known background is 23%, and conclude
"this analysis cannot be interpreted as significant
evidence for WIMP interactions, but we cannot
reject either event as signal."
More recently, on 4 September 2011, researchers
using the CRESST detectors presented evidence
of 67 collisions occurring in detector crystals
from sub-atomic particles, calculating there
is a less than 1 in 10,000 chance that all
were caused by known sources of interference
or contamination. It is quite possible then
that many of these collisions were caused
by WIMPs, and/or other unknown particles.
Indirect detection experiments
Indirect detection experiments search for
the products of WIMP annihilation or decay.
If WIMPs are Majorana particles then two WIMPs
could annihilate to produce gamma rays or
Standard Model particle-antiparticle pairs.
Additionally, if the WIMP is unstable, WIMPs
could decay into standard model particles.
These processes could be detected indirectly
through an excess of gamma rays, antiprotons
or positrons emanating from regions of high
dark matter density. The detection of such
a signal is not conclusive evidence for dark
matter, as the production of gamma rays from
other sources is not fully understood.
The EGRET gamma ray telescope observed more
gamma rays than expected from the Milky Way,
but scientists concluded that this was most
likely due to a mis-estimation of the telescope's
sensitivity.
The Fermi Gamma-ray Space Telescope, launched
11 June 2008, is searching for gamma rays
from dark matter annihilation and decay. In
April 2012, an analysis of previously available
data from its Large Area Telescope instrument
produced strong statistical evidence of a
130 GeV line in the gamma radiation coming
from the center of the Milky Way. At the time,
WIMP annihilation was the most probable explanation
for that line.
At higher energies, ground-based gamma-ray
telescopes have set limits on the annihilation
of dark matter in dwarf spheroidal galaxies
and in clusters of galaxies.
The PAMELA experiment has detected a larger
number of positrons than expected. These extra
positrons could be produced by dark matter
annihilation, but may also come from pulsars.
No excess of anti-protons has been observed.
The Alpha Magnetic Spectrometer on the International
Space Station is designed to directly measure
the fraction of cosmic rays which are positrons.
The first results, published in April 2013,
indicate an excess of high-energy cosmic rays
which could potentially be due to annihilation
of dark matter.
A few of the WIMPs passing through the Sun
or Earth may scatter off atoms and lose energy.
This way a large population of WIMPs may accumulate
at the center of these bodies, increasing
the chance that two will collide and annihilate.
This could produce a distinctive signal in
the form of high-energy neutrinos originating
from the center of the Sun or Earth. It is
generally considered that the detection of
such a signal would be the strongest indirect
proof of WIMP dark matter. High-energy neutrino
telescopes such as AMANDA, IceCube and ANTARES
are searching for this signal.
WIMP annihilation from the Milky Way Galaxy
as a whole may also be detected in the form
of various annihilation products. The Galactic
center is a particularly good place to look
because the density of dark matter may be
very high there.
In 2014, two independent and separate groups,
one led by the Leiden astrophysicist Alexey
Boyarsky and another from Harvard reported
an unidentified X-ray emission line around
3.5 keV in the spectra of clusters of galaxies;
it is possible this could be an indirect signal
from dark matter and that it could be a new
particle, a sterile neutrino which has mass.
Alternative theories
Numerous alternatives have been proposed to
explain these observations without the need
for a large amount of undetected matter. Most
of these modify the laws of gravity established
by Newton and Einstein in some way.
Modified gravity laws
The earliest modified gravity model to emerge
was Mordehai Milgrom's Modified Newtonian
Dynamics in 1983, which adjusts Newton's laws
to create a stronger gravitational field when
gravitational acceleration levels become tiny.
It had some success explaining galactic scale
features, such as rotational velocity curves
of elliptical galaxies, and dwarf elliptical
galaxies, but did not successfully explain
galaxy cluster gravitational lensing. However,
MOND was not relativistic, since it was just
a straight adjustment of the older Newtonian
account of gravitation, not of the newer account
in Einstein's general relativity. Soon after
1983, attempts were made to bring MOND into
conformity with General Relativity; this is
an ongoing process, and many competing hypotheses
have emerged based around the original MOND
model—including TeVeS, MOG or STV gravity,
and phenomenological covariant approach, among
others.
In 2007, John W. Moffat proposed a modified
gravity hypothesis based on the Nonsymmetric
Gravitational Theory that claims to account
for the behavior of colliding galaxies. This
model requires the presence of non-relativistic
neutrinos, or other candidates for dark matter,
to work.
Another proposal uses a gravitational backreaction
in an emerging theoretical field that seeks
to explain gravity between objects as an action,
a reaction, and then a back-reaction. Simply,
an object A affects an object B, and the object
B then re-affects object A, and so on: creating
a sort of feedback loop that strengthens gravity.
Recently, another group has proposed a modification
of large scale gravity in a hypothesis named
"dark fluid". In this formulation, the attractive
gravitational effects attributed to dark matter
are instead a side-effect of dark energy.
Dark fluid combines dark matter and dark energy
in a single energy field that produces different
effects at different scales. This treatment
is a simplified approach to a previous fluid-like
model called the Generalized Chaplygin gas
model where the whole of spacetime is a compressible
gas. Dark fluid can be compared to an atmospheric
system. Atmospheric pressure causes air to
expand, but part of the air can collapse to
form clouds. In the same way, the dark fluid
might generally expand, but it also could
collect around galaxies to help hold them
together.
Another set of proposals is based on the possibility
of a double metric tensor for space-time.
It has been argued that time-reversed solutions
in general relativity require such double
metric for consistency, and that both Dark
Matter and Dark Energy can be understood in
terms of time-reversed solutions of general
relativity.
Popular culture
Mention of dark matter is made in some video
games and other works of fiction. In such
cases, it is usually attributed extraordinary
physical or magical properties. Such descriptions
are often inconsistent with the properties
of dark matter proposed in physics and cosmology.
See also
References
External links
Dark matter at DMOZ
What is dark matter? at cosmosmagazine.com
The Dark Matter Crisis
The European astroparticle physics network
Helmholtz Alliance for Astroparticle Physics
A nice animation about dark matter
"NASA Finds Direct Proof of Dark Matter".
NASA. 21 August 2006. 
Tuttle, Kelen. "Dark Matter Observed". SLAC
Today. 
"Astronomers claim first 'dark galaxy' find".
New Scientist. 23 February 2005. 
Sample, Ian. "Dark Matter Detected". London:
Guardian. Retrieved 1 May 2010. 
Video lecture on dark matter by Scott Tremaine,
IAS professor
Science Daily story "Astronomers' Doubts About
the Dark Side ..."
Gray, Meghan; Merrifield, Mike; Copeland,
Ed. "Dark Matter". Sixty Symbols. Brady Haran
for the University of Nottingham. 
