Think about all of the stuff in the universe,
from the particles of dust on your screen,
to the billions of planets and galaxies you
see in the night sky. Everything you could
possibly imagine - all the observable matter
in the universe - is only 5% of what’s really
out there. Everything else is dark.
A property of most “normal” matter is
that it interacts with electromagnetic radiation,
or light. This means that when light reaches
an object it is typically absorbed or reflected.
This “interaction” allows us to detect
this matter. Dark matter, on the other hand,
doesn’t interact with light at all, rendering
it “invisible”. So if we can’t see dark
matter, what makes scientists believe it’s
there?
It all has to do with another property of
matter - the effect it has on other matter
in the universe. We call this phenomenon gravity,
and experience it as pull that the mass of
the sun exerts on the planets around it, keeping
the earth in orbit. When any two objects orbit
each other, this gravitational pull is what
keeps them from flying apart. The faster two
objects are orbiting each other, the more
mass we expect them to have.
In 1933, astronomer Fritz Zwicky noted something
extraordinary while studying a cluster of
galaxies. He measured how fast the galaxies
were moving around each other and used this
to estimate their mass. When he compared his
estimate to the observable mass of the galaxies,
he found that it was 400 times larger! He
argued that there must be some “missing”
or “invisible” mass and called it dark
matter allowing them to “orbit” so fast
without flying apart. This phenomenon has
since been observed and verified by other
scientists in many different galaxies including
our own!
But this speedy orbiting isn’t the only
reason scientists believe dark matter exists.
Another phenomenon related to mass is that
it affects the way light moves through the
universe. In a phenomenon called gravitational
lensing, light bends around massive objects
as it travels near them through space - the
more massive the object, the greater the bending.
By observing the light coming from distant
stars, scientists have measured stronger than
expected bending around galaxies, indicating
that the galaxies have more mass than we can
see - more evidence for this “missing”
matter.
So what is it? Well, since we can’t observe
it, scientists aren’t really sure. But there
are a number of theories as to what it might
be.
One theory is that dark matter is made of
Massive Compact Halo Objects, or MACHOs. MACHOs
are very large (up to thousands of times larger
than the sun) collections of “normal”
matter that do not typically interact with
light, and thus are not “visible”. Physicists
“observe” these objects through other
phenomena related to their gravity. Black
holes are an example of such objects, and
can only be detected by the way light and
matter collect around them. Unfortunately,
black holes and other MACHOs seem to exist
only in distinct pockets in the universe,
whereas observations show that dark matter
is spread out more evenly in space, so many
physicists reject the theory.
Instead, many physicists believe that undiscovered
particles are what constitutes dark matter.
Theory in other parts of physics requires
the existence of a special group of particles
called WIMPs - Weakly Interacting Massive
Particles. Scientists expect them to have
certain properties - mass and an inability
to interact with light - which fit well with
the description of dark matter, but since
we can’t observe them, we can’t be certain
that they’re there.
Physicists are trying to detect dark matter
through experiments on earth and observations
of the universe, but even with modern advancements
in science, we haven’t been able to pinpoint
exactly what dark matter is. The search for
the missing matter remains one of the biggest
unsolved problems in physics, and it waits
for someone to solve it.
