Dark matter is a hypothetical form of matter
that is thought to account for approximately
85% of the matter in the universe, and about
a quarter of its total energy density.
The majority of dark matter is thought to
be non-baryonic in nature, possibly being
composed of some as-yet undiscovered subatomic
particles.
Its presence is implied in a variety of astrophysical
observations, including gravitational effects
that cannot be explained unless more matter
is present than can be seen.
For this reason, most experts think dark matter
to be ubiquitous in the universe and to have
had a strong influence on its structure and
evolution.
The name dark matter refers to the fact that
it does not appear to interact with observable
electromagnetic radiation, such as light,
and is thus invisible (or 'dark') to the entire
electromagnetic spectrum, making it extremely
difficult to detect using usual astronomical
equipment.The primary evidence for dark matter
is that calculations show that many galaxies
would fly apart instead of rotating, or would
not have formed or move as they do, if they
did not contain a large amount of unseen matter.
Other lines of evidence include observations
in gravitational lensing, from the cosmic
microwave background, from astronomical observations
of the observable universe's current structure,
from the formation and evolution of galaxies,
from mass location during galactic collisions,
and from the motion of galaxies within galaxy
clusters.
In the standard Lambda-CDM model of cosmology,
the total mass–energy of the universe contains
5% ordinary matter and energy, 27% dark matter
and 68% of an unknown form of energy known
as dark energy.
Thus, dark matter constitutes 85% of total
mass, while dark energy plus dark matter constitute
95% of total mass–energy content.Because
dark matter has not yet been observed directly,
it must barely interact with ordinary baryonic
matter and radiation.
The primary candidate for dark matter is some
new kind of elementary particle that has not
yet been discovered, in particular, weakly-interacting
massive particles (WIMPs), or gravitationally-interacting
massive particles (GIMPs).
Many experiments to directly detect and study
dark matter particles are being actively undertaken,
but none has yet succeeded.
Dark matter is classified as cold, warm, or
hot according to its velocity (more precisely,
its free streaming length).
Current models favor a cold dark matter scenario,
in which structures emerge by gradual accumulation
of particles.
Although the existence of dark matter is generally
accepted by the scientific community, some
astrophysicists, intrigued by certain observations
that do not fit the dark matter theory, argue
for various modifications of the standard
laws of general relativity, such as modified
Newtonian dynamics, tensor–vector–scalar
gravity, or entropic gravity.
These models attempt to account for all observations
without invoking supplemental non-baryonic
matter.
== History ==
=== 
Early history ===
The hypothesis of dark matter has an elaborate
history.
In a talk given in 1884, Lord Kelvin estimated
the number of dark bodies in the Milky Way
from the observed velocity dispersion of the
stars orbiting around the center of the galaxy.
By using these measurements, he estimated
the mass of the galaxy, which he determined
is different from the mass of visible stars.
Lord Kelvin thus concluded that "many of our
stars, perhaps a great majority of them, may
be dark bodies".
In 1906 Henri Poincaré in "The Milky Way
and Theory of Gases" used "dark matter", or
"matière obscure" in French, in discussing
Kelvin's work.The first to suggest the existence
of dark matter, using stellar velocities,
was Dutch astronomer Jacobus Kapteyn in 1922.
Fellow Dutchman and radio astronomy pioneer
Jan Oort also hypothesized the existence of
dark matter in 1932.
Oort was studying stellar motions in the local
galactic neighborhood and found that the mass
in the galactic plane must be greater than
what was observed, but this measurement was
later determined to be erroneous.In 1933,
Swiss astrophysicist Fritz Zwicky, who studied
galaxy clusters while working at the California
Institute of Technology, made a similar inference.
Zwicky applied the virial theorem to the Coma
Cluster and obtained evidence of unseen mass
that he called dunkle Materie ('dark matter').
Zwicky estimated its mass based on the motions
of galaxies near its edge and compared that
to an estimate based on its brightness and
number of galaxies.
He estimated that the cluster had about 400
times more mass than was visually observable.
The gravity effect of the visible galaxies
was far too small for such fast orbits, thus
mass must be hidden from view.
Based on these conclusions, Zwicky inferred
that some unseen matter provided the mass
and associated gravitation attraction to hold
the cluster together.
This was the first formal inference about
the existence of dark matter.
Zwicky's estimates were off by more than an
order of magnitude, mainly due to an obsolete
value of the Hubble constant; the same calculation
today shows a smaller fraction, using greater
values for luminous mass.
However, Zwicky did correctly infer that the
bulk of the matter was dark.The first robust
indications that the mass-to-light ratio was
anything other than unity came from measurements
of galaxy rotation curves.
In 1939, Horace W. Babcock reported the rotation
curve for the Andromeda nebula (known now
as the Andromeda Galaxy), which suggested
that the mass-to-luminosity ratio increases
radially.
He attributed it to either light absorption
within the galaxy or modified dynamics in
the outer portions of the spiral and not to
missing matter.
=== 1970s ===
Vera Rubin and Kent Ford in the 1960s and
1970s provided further strong evidence, also
using galaxy rotation curves.
Rubin worked with a new spectrograph to measure
the velocity curve of edge-on spiral galaxies
with greater accuracy.
This result was confirmed in 1978.
An influential paper presented Rubin's results
in 1980.
Rubin found that most galaxies must contain
about six times as much dark as visible mass;
thus, by around 1980 the apparent need for
dark matter was widely recognized as a major
unsolved problem in astronomy.At the same
time that Rubin and Ford were exploring optical
rotation curves, radio astronomers were making
use of new radio telescopes to map the 21
cm line of atomic hydrogen in nearby galaxies.
The radial distribution of interstellar atomic
hydrogen (HI) often extends to much larger
galactic radii than those accessible by optical
studies, allowing the sampling of rotation
curves—and thus of the total mass distribution—to
a new dynamical regime.
Early mapping of Andromeda with the 300-foot
telescope at Green Bank and the 250-foot dish
at Jodrell Bank already showed that the HI
rotation curve did not trace the expected
Keplerian decline.
As more sensitive receivers became available,
Morton Roberts and Robert Whitehurst were
able to trace the rotational velocity of Andromeda
to 30 kpc, much beyond the optical measurements.
Illustrating the advantage of tracing the
gas disk at large radii, Figure 16 of that
paper combines the optical data (the cluster
of points at radii of less than 15 kpc with
a single point further out) with the HI data
between 20 and 30 kpc, exhibiting the flatness
of the outer galaxy rotation curve; the solid
curve peaking at the center is the optical
surface density, while the other curve shows
the cumulative mass, still rising linearly
at the outermost measurement.
In parallel, the use of interferometric arrays
for extragalactic HI spectroscopy was being
developed.
In 1972, David Rogstad and Seth Shostak published
HI rotation curves of five spirals mapped
with the Owens Valley interferometer; the
rotation curves of all five were very flat,
suggesting very large values of mass-to-light
ratio in the outer parts of their extended
HI disks.
A stream of observations in the 1980s supported
the presence of dark matter, including gravitational
lensing of background objects by galaxy clusters,
the temperature distribution of hot gas in
galaxies and clusters, and 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.
== Technical definition ==
In standard cosmology, matter is anything
whose energy density scales with the inverse
cube of the scale factor, i.e., ρ ∝ a−3.
This is in contrast to radiation, which scales
as the inverse fourth power of the scale factor
ρ ∝ a−4 , and a cosmological constant,
which is independent of a.
These scalings can be understood intuitively:
for an ordinary particle in a cubical box,
doubling the length of the sides of the box
decreases the density (and hence energy density)
by a factor of eight (23).
For radiation, the decrease in energy density
is larger because an increase in scale factor
causes a proportional redshift.
A cosmological constant, as an intrinsic property
of space, has a constant energy density regardless
of the volume under consideration.In principle,
"dark matter" means all components of the
universe that are not visible but still obey
ρ ∝ a−3.
In practice, the term "dark matter" is often
used to mean only the non-baryonic component
of dark matter, i.e., excluding "missing baryons."
Context will usually indicate which meaning
is intended.
== Observational evidence ==
=== Galaxy rotation curves ===
The arms of spiral galaxies rotate around
the galactic center.
The luminous mass density of a spiral galaxy
decreases as one goes from the center to the
outskirts.
If luminous mass were all the matter, then
we can model the galaxy as a point mass in
the centre and test masses orbiting around
it, similar to the Solar System.
From Kepler's Second Law, it is expected that
the rotation velocities will decrease with
distance from the center, similar to the Solar
System.
This is not observed.
Instead, the galaxy rotation curve remains
flat as distance from the center increases.
If Kepler's laws are correct, then the obvious
way to resolve this discrepancy is to conclude
that the mass distribution in spiral galaxies
is not similar to that of the Solar System.
In particular, there is a lot of non-luminous
matter (dark matter) in the outskirts of the
galaxy.
=== Velocity dispersions ===
Stars in bound systems must obey the virial
theorem.
The theorem, together with the measured velocity
distribution, can be used to measure the mass
distribution in a bound system, such as elliptical
galaxies or globular clusters.
With some exceptions, velocity dispersion
estimates of elliptical galaxies do not match
the predicted velocity dispersion from the
observed mass distribution, even assuming
complicated distributions of stellar orbits.As
with galaxy rotation curves, the obvious way
to resolve the discrepancy is to postulate
the existence of non-luminous matter.
=== Galaxy clusters ===
Galaxy clusters are particularly 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 clusters
From X-rays emitted by hot gas in the clusters.
From the X-ray energy spectrum and flux, the
gas temperature and density can be estimated,
hence giving the pressure; assuming pressure
and gravity balance determines the cluster's
mass profile.
Gravitational lensing (usually of more distant
galaxies) can measure cluster masses without
relying on observations of dynamics (e.g.,
velocity).Generally, these three methods are
in reasonable agreement that dark matter outweighs
visible matter by approximately 5 to 1.
=== Gravitational lensing ===
One of the consequences of general relativity
is that massive objects (such as a cluster
of galaxies) lying between a more distant
source (such as a quasar) and an observer
should act as a lens to bend the light from
this source.
The more massive an object, the more lensing
is observed.
Strong lensing is the observed distortion
of background galaxies into arcs when their
light passes through such a gravitational
lens.
It has been observed around many distant clusters
including Abell 1689.
By measuring the distortion geometry, the
mass of the intervening cluster 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.
Lensing can lead to multiple copies of an
image.
By analyzing the distribution of multiple
image copies, scientists have been able to
deduce and map the distribution of dark matter
around the MACS J0416.1-2403 galaxy cluster.Weak
gravitational lensing investigates minute
distortions of galaxies, using statistical
analyses from vast galaxy surveys.
By examining the apparent shear deformation
of the adjacent background galaxies, the mean
distribution of dark matter can be characterized.
The mass-to-light ratios correspond to dark
matter densities predicted by other large-scale
structure measurements.
Dark matter does not bend light itself; mass
(in this case the mass of the dark matter)
bends spacetime.
Light follows the curvature of spacetime,
resulting in the lensing effect.
=== Cosmic microwave background ===
Although both dark matter and ordinary matter
are matter, they do not behave in the same
way.
In particular, in the early universe, ordinary
matter was ionized and interacted strongly
with radiation via Thomson scattering.
Dark matter does not interact directly with
radiation, but it does affect the CMB by its
gravitational potential (mainly on large scales),
and by its effects on the density and velocity
of ordinary matter.
Ordinary and dark matter perturbations, therefore,
evolve differently with time and leave different
imprints on the cosmic microwave background
(CMB).
The cosmic microwave background is very close
to a perfect blackbody but contains very small
temperature anisotropies of a few parts in
100,000.
A sky map of anisotropies can be decomposed
into an angular power spectrum, which is observed
to contain a series of acoustic peaks at near-equal
spacing but different heights.
The series of peaks can be predicted for any
assumed set of cosmological parameters by
modern computer codes such as CMBFast and
CAMB, and matching theory to data, therefore,
constrains cosmological parameters.
The first peak mostly shows the density of
baryonic matter, while the third peak relates
mostly to the density of dark matter, measuring
the density of matter and the density of atoms.The
CMB anisotropy was first discovered by COBE
in 1992, though this had too coarse resolution
to detect the acoustic peaks.
After the discovery of the first acoustic
peak by the balloon-borne BOOMERanG experiment
in 2000,
the power spectrum was precisely observed
by WMAP in 2003-12, and even more precisely
by the Planck spacecraft in 2013-15.
The results support the Lambda-CDM model.The
observed CMB angular power spectrum provides
powerful evidence in support of dark matter,
as its precise structure is well fitted by
the Lambda-CDM model, but difficult to reproduce
with any competing model such as modified
Newtonian dynamics (MOND).
=== Structure formation ===
Structure formation refers to the period after
the Big Bang when density perturbations collapsed
to form stars, galaxies, and clusters.
Prior to structure formation, the Friedmann
solutions to general relativity describe a
homogeneous universe.
Later, small anisotropies gradually grew and
condensed the homogeneous universe into stars,
galaxies and larger structures.
Ordinary matter is affected by radiation,
which is the dominant element of the universe
at very early times.
As a result, its density perturbations are
washed out and unable to condense into structure.
If there were only ordinary matter in the
universe, there would not have been enough
time for density perturbations to grow into
the galaxies and clusters currently seen.
Dark matter provides a solution to this problem
because it is unaffected by radiation.
Therefore, its density perturbations can grow
first.
The resulting gravitational potential acts
as an attractive potential well for ordinary
matter collapsing later, speeding up the structure
formation process.
=== Bullet Cluster ===
If dark matter does not exist, then the next
most likely explanation is that general relativity—the
prevailing theory of gravity—is incorrect.
The Bullet Cluster, the result of a recent
collision of two galaxy clusters, provides
a challenge for modified gravity theories
because its apparent center of mass is far
displaced from the baryonic center of mass.
Standard dark matter theory can easily explain
this observation, but modified gravity has
a much harder time, especially since the observational
evidence is model-independent.
=== 
Type Ia supernova distance measurements ===
Type Ia supernovae can be used as standard
candles to measure extragalactic distances,
which can in turn be used to measure how fast
the universe has expanded in the past.
The data indicates that the universe is expanding
at an accelerating rate, the cause of which
is usually ascribed to dark energy.
Since observations indicate the universe is
almost flat, it is expected that the total
energy density of everything in the universe
to sum to 1 (Ωtot ~ 1).
The measured dark energy density 
is ΩΛ = ~0.690; the observed ordinary (baryonic)
matter energy density is Ωb = ~0.0482 and
the energy density of radiation is negligible.
This leaves a missing Ωdm = ~0.258 that nonetheless
behaves like matter (see technical definition
section above)—dark matter.
=== Sky surveys and baryon acoustic oscillations
===
Baryon acoustic oscillations (BAO) are fluctuations
in the density of the visible baryonic matter
(normal matter) of the universe on large scales.
These are predicted to arise in the Lambda-CDM
model due to acoustic oscillations in the
photon-baryon fluid of the early universe,
and can be observed in the cosmic microwave
background angular power spectrum.
BAOs set up a preferred length scale for baryons.
As the dark matter and baryons clumped together
after recombination, the effect is much weaker
in the galaxy distribution in the nearby universe,
but is detectable as a subtle (~ 1 percent)
preference for pairs of galaxies to be separated
by 147 Mpc, compared to those separated by
130 or 160 Mpc.
This feature was predicted theoretically in
the 1990s and then discovered in 2005, in
two large galaxy redshift surveys, the Sloan
Digital Sky Survey and the 2dF Galaxy Redshift
Survey.
Combining the CMB observations with BAO measurements
from galaxy redshift surveys provides a precise
estimate of the Hubble constant and the average
matter density in the Universe.
The results support the Lambda-CDM model.
=== Redshift-space distortions ===
Large galaxy redshift surveys may be used
to make a three-dimensional map of the galaxy
distribution.
These maps are slightly distorted because
distances are estimated from observed redshifts;
the redshift contains a contribution from
the galaxy's so-called peculiar velocity in
addition to the dominant Hubble expansion
term.
On average, superclusters are expanding but
more slowly than the cosmic mean due to their
gravity, while voids are expanding faster
than average.
In a redshift map, galaxies in front of a
supercluster have excess radial velocities
towards it and have redshifts slightly higher
than their distance would imply, while galaxies
behind the supercluster have redshifts slightly
low for their distance.
This effect causes superclusters to appear
squashed in the radial direction, and likewise
voids are stretched.
Their angular positions are unaffected.
The effect is not detectable for any one structure
since the true shape is not known, but can
be measured by averaging over many structures
assuming Earth is not at a special location
in the Universe.
The effect was predicted quantitatively by
Nick Kaiser in 1987, and first decisively
measured in 2001 by the 2dF Galaxy Redshift
Survey.
Results are in agreement with the Lambda-CDM
model.
=== Lyman-alpha forest ===
In astronomical spectroscopy, the Lyman-alpha
forest is the sum of the absorption lines
arising from the Lyman-alpha transition of
neutral hydrogen in the spectra of distant
galaxies and quasars.
Lyman-alpha forest observations can also constrain
cosmological models.
These constraints agree with those obtained
from WMAP data.
== Composition of dark matter: baryonic vs.
nonbaryonic ==
Dark matter can refer to any substance that
interacts predominantly via gravity with visible
matter (e.g., stars and planets).
Hence in principle it need not be composed
of a new type of fundamental particle but
could, at least in part, be made up of standard
baryonic matter, such as protons or neutrons.
However, for the reasons outlined below, most
scientists think the dark matter is dominated
by a non-baryonic component, which is likely
composed of a currently unknown fundamental
particle (or similar exotic state).
=== Baryonic matter ===
Baryons (protons and neutrons) make up ordinary
stars and planets.
However, baryonic matter also encompasses
less common black holes, neutron stars, faint
old white dwarfs and brown dwarfs, collectively
known as massive compact halo objects (MACHOs),
which can be hard to detect.However, multiple
lines of evidence suggest the majority of
dark matter is not made of baryons:
Sufficient diffuse, baryonic gas or dust would
be visible when backlit by stars.
The theory of Big Bang nucleosynthesis predicts
the observed abundance of the chemical elements.
If there are more baryons, then there should
also be more helium, lithium and heavier elements
synthesized during the Big Bang.
Agreement with observed abundances requires
that baryonic matter makes up between 4–5%
of the universe's critical density.
In contrast, large-scale structure and other
observations indicate that the total matter
density is about 30% of the critical density.
Astronomical searches for gravitational microlensing
in the Milky Way found that at most a small
fraction of the dark matter may be in dark,
compact, conventional objects (MACHOs, etc.);
the excluded range of object masses is from
half the Earth's mass up to 30 solar masses,
which covers nearly all the plausible candidates.
Detailed analysis of the small irregularities
(anisotropies) in the cosmic microwave background.
Observations by WMAP and Planck indicate that
around five sixths of the total matter is
in a form that interacts significantly with
ordinary matter or photons only through gravitational
effects.
=== Non-baryonic matter ===
Candidates for non-baryonic dark matter are
hypothetical particles such as axions, sterile
neutrinos, weakly interacting massive particles
(WIMPs), gravitationally-interacting massive
particles (GIMPs), or supersymmetric particles.
The three neutrino types already observed
are indeed abundant, and dark, and matter,
but because their individual masses—however
uncertain they may be—are almost certainly
tiny, they can only supply a small fraction
of dark matter, due to limits derived from
large-scale structure and high-redshift galaxies.Unlike
baryonic matter, nonbaryonic matter did not
contribute to the formation of the elements
in the early universe (Big Bang nucleosynthesis)
and so its presence is revealed only via its
gravitational effects, or weak lensing.
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 (indirect
detection).
=== Dark matter aggregation and dense dark
matter objects ===
If dark matter is as common as observations
suggest, an obvious question is whether it
can form objects equivalent to planets, stars,
or black holes.
The answer has historically been that it cannot,
because of two factors:
It lacks an efficient means to lose energy:
Ordinary matter forms dense objects because
it has numerous ways to lose energy.
Losing energy would be essential for object
formation, because a particle that gains energy
during compaction or falling "inward" under
gravity, and cannot lose it any other way,
will heat up and increase velocity and momentum.
Dark matter appears to lack means to lose
energy, simply because it is not capable of
interacting strongly in other ways except
through gravity.
The Virial theorem suggests that such a particle
would not stay bound to the gradually forming
object—as the object began to form and compact,
the dark matter particles within it would
speed up and tend to escape.
It lacks a range of interactions needed to
form structures: Ordinary matter interacts
in many different ways.
This allow it to form more complex structures.
For example, stars form through gravity, but
the particles within them interact and can
emit energy in the form of neutrinos and electromagnetic
radiation through fusion when they become
energetic enough.
Protons and neutrons can bind via the strong
interaction and then form atoms with electrons
largely through electromagnetic interaction.
But there is no evidence that dark matter
is capable of such a wide variety of interactions,
since it only seems to interact through gravity
and through some means no stronger than the
weak interaction (although this is speculative
until dark matter is better understood).This
question has been debated heavily during recent
years.
In 2016–2017 the idea of dense dark matter
or dark matter being black holes, including
primordial black holes, made a comeback following
results of gravitation wave detection.
These were again ruled out in December 2017,
but research and theories based on these still
continue as at 2018, including approaches
to dark matter cooling, and the question is
by no means settled.
== Classification of dark matter: cold, warm
or hot ==
Dark matter can be divided into cold, warm,
and hot categories.
These categories refer to velocity rather
than an actual temperature, indicating how
far corresponding objects moved due to random
motions in the early universe, before they
slowed due to cosmic expansion—this is an
important distance called the free streaming
length (FSL).
Primordial density fluctuations smaller than
this length get washed out as particles spread
from overdense to underdense regions, while
larger fluctuations are unaffected; therefore
this length sets a minimum scale for later
structure formation.
The categories are set with respect to the
size of a protogalaxy (an object that later
evolves into a dwarf galaxy): dark matter
particles are classified as cold, warm, or
hot according as their FSL; much smaller (cold),
similar (warm), or much larger (hot) than
a protogalaxy.Mixtures of the above are also
possible: a theory of mixed dark matter was
popular in the mid-1990s, but was rejected
following the discovery of dark energy.Cold
dark matter leads to a bottom-up formation
of structure while hot dark matter would result
in a top-down formation scenario; the latter
is excluded by high-redshift galaxy observations.
=== Alternative definitions ===
These categories also correspond to fluctuation
spectrum effects and the interval following
the Big Bang at which each type became non-relativistic.
Davis et al. wrote in 1985:
Candidate particles can be grouped into three
categories on the basis of their effect on
the fluctuation spectrum (Bond et al. 1983).
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
(Pagels and Primack 1982; Bond, Szalay and
Turner 1982) ... Any particles which became
nonrelativistic very early, and so were able
to diffuse a negligible distance, are termed
"cold" dark matter (CDM).
There are many candidates for CDM including
supersymmetric particles.
Another approximate dividing line is that
warm dark matter became non-relativistic when
the universe was approximately 1 year old
and 1 millionth of its present size and in
the radiation-dominated era (photons and neutrinos),
with a photon temperature 2.7 million K. Standard
physical cosmology gives the particle horizon
size as 2ct (speed of light multiplied by
time) in the radiation-dominated era, thus
2 light-years.
A region of this size would expand to 2 million
light-years today (absent structure formation).
The actual FSL is approximately 5 times the
above length, since it continues to grow slowly
as particle velocities decrease inversely
with the scale factor after they become non-relativistic.
In this example the FSL would correspond to
10 million light-years, or 3 megaparsecs,
today, around the size containing an average
large galaxy.
The 2.7 million K photon temperature gives
a typical photon energy of 250 electron-volts,
thereby setting 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
one year after the Big Bang and thus have
FSLs much smaller than a protogalaxy, making
them cold.
Conversely, much lighter particles, such as
neutrinos with masses of only a few eV, have
FSLs much larger than a protogalaxy, thus
qualifying them as hot.
=== Cold dark matter ===
Cold dark matter offers the simplest explanation
for most cosmological observations.
It is dark matter composed of constituents
with an FSL much smaller than a protogalaxy.
This is the focus for dark matter research,
as hot dark matter does not seem capable of
supporting galaxy or galaxy cluster formation,
and most particle candidates slowed early.
The constituents of cold dark matter are unknown.
Possibilities range from large objects like
MACHOs (such as black holes and Preon stars)
or RAMBOs (such as clusters of brown dwarfs),
to new particles such as WIMPs and axions.
Studies of Big Bang nucleosynthesis and gravitational
lensing convinced most cosmologists that MACHOs
cannot make up more than a small fraction
of dark matter.
According to A. Peter: "... the only really
plausible dark-matter candidates are new particles."
Specifically, Jamie Farnes proposes a particle
with negative mass.
The 1997 DAMA/NaI experiment and its successor
DAMA/LIBRA in 2013, claimed to directly detect
dark matter particles passing through the
Earth, but many researchers remain skeptical,
as negative results from similar experiments
seem incompatible with the DAMA results.
Many supersymmetric models offer dark matter
candidates in the form of the WIMPy Lightest
Supersymmetric Particle (LSP).
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 comprises particles with
an FSL comparable to the size of a protogalaxy.
Predictions based on warm dark matter are
similar to those for cold dark matter on large
scales, 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 a better fit
to observations.
A challenge for this model is the lack of
particle candidates with the required mass
~ 300 eV to 3000 eV.No known particles can
be categorized as warm dark matter.
A postulated candidate is the sterile neutrino:
a heavier, slower form of neutrino that does
not interact through the weak force, unlike
other neutrinos.
Some modified gravity theories, such as scalar–tensor–vector
gravity, require "warm" dark matter to make
their equations work.
=== Hot dark matter ===
Hot dark matter consists of particles whose
FSL is much larger than the size of a protogalaxy.
The neutrino qualifies as such particle.
They were discovered independently, long before
the hunt for dark matter: they were postulated
in 1930, and detected in 1956.
Neutrinos' mass is less than 10−6 that of
an electron.
Neutrinos interact with normal matter only
via gravity and the weak force, making them
difficult to detect (the weak force only works
over a small distance, thus a neutrino triggers
a weak force event only if it hits a nucleus
head-on).
This makes them 'weakly interacting light
particles' (WILPs), as opposed to WIMPs.
The three known flavours of neutrinos are
the electron, muon, and tau.
Their masses are slightly different.
Neutrinos oscillate among the flavours as
they move.
It is hard to determine an exact upper bound
on the collective average mass of the three
neutrinos (or for any of the three individually).
For example, if the average neutrino mass
were over 50 eV/c2 (less than 10−5 of the
mass of an electron), the universe would collapse.
CMB data and other methods indicate that their
average mass probably does not exceed 0.3
eV/c2.
Thus, observed neutrinos cannot explain dark
matter.Because galaxy-size density fluctuations
get washed out by free-streaming, hot dark
matter implies that the first objects that
can form are huge supercluster-size pancakes,
which then fragment into galaxies.
Deep-field observations show instead that
galaxies formed first, followed by clusters
and superclusters as galaxies clump together.
== Detection of dark matter particles ==
If dark matter is made up of sub-atomic particles,
then millions, possibly billions, of such
particles must pass through every square centimeter
of the Earth each second.
Many experiments aim to test this hypothesis.
Although WIMPs are popular search candidates,
the Axion Dark Matter Experiment (ADMX) searches
for axions.
Another candidate is heavy hidden sector particles
that 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
dark matter particle annihilations or decays.
=== Direct detection ===
Direct detection experiments aim to observe
low-energy recoils (typically a few keVs)
of nuclei induced by interactions with particles
of dark matter, which (in theory) are passing
through the Earth.
After such a recoil the nucleus will emit
energy as, e.g., scintillation light or phonons,
which is then detected by sensitive apparatus.
To do this effectively, it is crucial to maintain
a low background, and so such experiments
operate deep underground to reduce the interference
from cosmic rays.
Examples of underground laboratories with
direct detection experiments include the Stawell
mine, the Soudan mine, the SNOLAB underground
laboratory at Sudbury, the Gran Sasso National
Laboratory, the Canfranc Underground Laboratory,
the Boulby Underground Laboratory, the Deep
Underground Science and Engineering Laboratory
and the China Jinping Underground Laboratory.
These experiments mostly use either cryogenic
or noble liquid detector technologies.
Cryogenic detectors operating at temperatures
below 100 mK, detect the heat produced when
a particle hits an atom in a crystal absorber
such as germanium.
Noble liquid detectors detect scintillation
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, PandaX, and LUX,
the Large Underground Xenon experiment.
Both of these techniques focus strongly on
their ability to distinguish background particles
(which predominantly scatter off electrons)
from dark matter particles (that scatter off
nuclei).
Other experiments include SIMPLE and PICASSO.
Currently there has been no well-established
claim of dark matter detection from a direct
detection experiment, leading instead to strong
upper limits on the mass and interaction cross
section with nucleons of such dark matter
particles.
The DAMA/NaI and more recent DAMA/LIBRA experimental
collaborations have detected an annual modulation
in the rate of events in their detectors,
which they claim is due to dark matter.
This results from the expectation that as
the Earth orbits the Sun, the velocity of
the detector relative to the dark matter halo
will vary by a small amount.
This claim is so far unconfirmed and in contradiction
with negative results from other experiments
such as LUX and SuperCDMS.A special case of
direct detection experiments covers those
with directional sensitivity.
This is a search strategy based on the motion
of the Solar System around the Galactic Center.
A low-pressure time projection chamber makes
it possible to access information on recoiling
tracks and constrain WIMP-nucleus kinematics.
WIMPs coming from the direction in which the
Sun travels (approximately towards Cygnus)
may then be separated from background, which
should be isotropic.
Directional dark matter experiments include
DMTPC, DRIFT, Newage and MIMAC.
=== Indirect detection ===
Indirect detection experiments search for
the products of the self-annihilation or decay
of dark matter particles in outer space.
For example, in regions of high dark matter
density (e.g., the centre of our galaxy) two
dark matter particles could annihilate to
produce gamma rays or Standard Model particle-antiparticle
pairs.
Alternatively if the dark matter particle
is unstable, it could decay into standard
model (or other) particles.
These processes could be detected indirectly
through an excess of gamma rays, antiprotons
or positrons emanating from high density regions
in our galaxy or others.
A major difficulty inherent in such searches
is that various astrophysical sources can
mimic the signal expected from dark matter,
and so multiple signals are likely required
for a conclusive discovery.A few of the dark
matter particles passing through the Sun or
Earth may scatter off atoms and lose energy.
Thus dark matter may accumulate at the center
of these bodies, increasing the chance of
collision/annihilation.
This could produce a distinctive signal in
the form of high-energy neutrinos.
Such a signal would be strong indirect proof
of WIMP dark matter.
High-energy neutrino telescopes such as AMANDA,
IceCube and ANTARES are searching for this
signal.
The detection by LIGO in September 2015 of
gravitational waves, opens the possibility
of observing dark matter in a new way, particularly
if it is in the form of primordial black holes.Many
experimental searches have been undertaken
to look for such emission from dark matter
annihilation or decay, examples of which follow.
The Energetic Gamma Ray Experiment Telescope
observed more gamma rays in 2008 than expected
from the Milky Way, but scientists concluded
that this was most likely due to incorrect
estimation of the telescope's sensitivity.The
Fermi Gamma-ray Space Telescope is searching
for similar gamma rays.
In April 2012, an analysis of previously available
data from its Large Area Telescope instrument
produced statistical evidence of a 130 GeV
signal in the gamma radiation coming from
the center of the Milky Way.
WIMP annihilation was seen as the most probable
explanation.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 (launched in 2006) detected excess
positrons.
They could be from dark matter annihilation
or from pulsars.
No excess antiprotons were observed.In 2013
results from the Alpha Magnetic Spectrometer
on the International Space Station indicated
excess high-energy cosmic rays that could
be due to dark matter annihilation.
=== Collider searches for dark matter ===
An alternative approach to the detection of
dark matter particles in nature is to produce
them in a laboratory.
Experiments with the Large Hadron Collider
(LHC) may be able to detect dark matter particles
produced in collisions of the LHC proton beams.
Because a dark matter particle should have
negligible interactions with normal visible
matter, it may be detected indirectly as (large
amounts of) missing energy and momentum that
escape the detectors, provided other (non-negligible)
collision products are detected.
Constraints on dark matter also exist from
the LEP experiment using a similar principle,
but probing the interaction of dark matter
particles with electrons rather than quarks.
It is important to note that any discovery
from collider searches must be corroborated
by discoveries in the indirect or direct detection
sectors to prove that the particle discovered
is, in fact, the dark matter of our Universe.
== Alternative hypotheses ==
Because dark matter remains to be conclusively
identified, many other hypotheses have emerged
aiming to explain the observational phenomena
that dark matter was conceived to explain.
The most common method is to modify general
relativity.
General relativity is well-tested on solar
system scales, but its validity on galactic
or cosmological scales has not been well proven.
A suitable modification to general relativity
can conceivably eliminate the need for dark
matter.
The best-known theories of this class are
MOND and its relativistic generalization tensor-vector-scalar
gravity (TeVeS), f(R) gravity and entropic
gravity.
Alternative theories abound.A problem with
alternative hypotheses is that the observational
evidence for dark matter comes from so many
independent approaches (see the "observational
evidence" section above).
Explaining any individual observation is possible
but explaining all of them is very difficult.
Nonetheless, there have been some scattered
successes for alternative hypotheses, such
as a 2016 test of gravitational lensing in
entropic gravity.The prevailing opinion among
most astrophysicists is that while modifications
to general relativity can conceivably explain
part of the observational evidence, there
is probably enough data to conclude there
must be some form of dark matter.
== In philosophy of science ==
In philosophy of science, dark matter is an
example of an auxiliary hypothesis, an ad
hoc postulate that is added to a theory in
response to observations that falsify it.
It has been argued that the dark matter hypothesis
is a conventionalist hypothesis, that is,
a hypothesis that adds no empirical content
and hence is unfalsifiable in the sense defined
by Karl Popper.
== In popular culture ==
Mention of dark matter is made in works of
fiction.
In such cases, it is usually attributed extraordinary
physical or magical properties.
Such descriptions are often inconsistent with
the hypothesized properties of dark matter
in physics and cosmology.
== See also ==
== Notes
