Dark matter is just a form of matter which is to say it acts like matter when it comes
to gravity. So it clumps together like the matter we know about. It’s found in galaxies
for example because of gravitational force. What distinguishes dark matter from ordinary
matter is that it has no interaction as far as we know with light. So we see ordinary
matter. It’s made up of atoms. Atoms are made up of charged particles. But so far as
we know dark matter is just an entirely new form of matter not made up of atoms, not made
up of the stuff we’re familiar with. And the question we eventually have is what is
it made up of exactly?
But as far as the physics of the universe goes it’s just a form of matter. The reason
we’re aware of dark matter is because of the gravitational effects. In fact if you
look at just the energy stored there’s five times as much dark matter as there is ordinary
matter. So you observe this gravitational effects in galaxies for example. I mean one
of the ways we first knew about dark matter was by looking at the motion of stars. The
motion of stars responds to the gravitational force of all the matter around. It doesn’t
care whether or not it interacts with light. The stars of course are bright because they
interact with light. But they’re responding to the gravity of the matter including the
dark matter. So that was evidence for dark matter. And now there’s lots of other evidence
for dark matter too having to do with the way light bends or what galaxy clusters look
like when they merge. So there’s really a lot of physical evidence that tells us dark
matter is out there in the universe. Then the question for theoretical physicists like
myself becomes what is this stuff and what do we mean by that? Well is it an elementary
particle? Is it more than one elementary particle? If it is a particle what is the mass of that
particle? Does it have any interactions at all?
So far we haven’t seen any interactions with the light with which we’re familiar
but maybe there’s a small interaction that we just haven’t seen yet or maybe it attracts
in an entirely different way. The only thing we know for sure is that there is this matter
and it interacts via gravity. Dark matter was actually essential to the formation of
structures we see in the lifetime of the universe. Now it’s important to say structures we
see in the lifetime of the universe. Even without dark matter structure would have formed.
But the actual size of the galaxies that we see is only possible because dark matter was
present. Ordinary matter interacts with radiation. Radiation would have washed away small objects
like galaxies. Now I realize galaxies don’t seem small to you but on the scale of what
radiation could wash away they’re actually small. So dark matter was essential to forming
objects of that size. It also was important because it meant that matter came to dominate
over radiation sooner in the evolution of the universe because there was a lot more
matter. And again matter domination is important for the formation of structure because radiation
won’t form structure. I mean just think about it. Light’s not clumping into little
balls the same way matter would. So both because of its abundance and because it doesn’t
interact with light, dark matter was actually essential to the formation of the structure
that we see.