Dark Matter: Radiation Would Have Washed Away Galaxies Without It, with Lisa Randall | Big Think

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 these 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.