Question: what are the basics of dark matter?

Baptiste Ravina answered on 12 Mar 2020:

Great question! We’ve known about Dark Matter for a while, from astrophysical measurements of galaxies. Let me try to explain.

Physicists were interested in measuring galaxies, and figuring out how stars moved about in those galaxies. If you have precise enough telescopes and you know about gravity (Newton’s laws to a good approximation, but also some general relativity), it’s something you can do. They measured the rotational velocities of stars in galaxies – that is, how fast they were orbiting depending on how far they were from the center of the galaxy – and expected to see something like a bell-shaped curve, with a peak (high velocities) near the center, and then a gradual decrease (lower velocities) as you move away from the center. This is the same kind of physics involved in the orbits of planets around the Sun, and it’s been pretty well established since Kepler. All you need are the masses of these stars, and that’s also something we know how to calculate: their mass is related to their brightness.

But in fact, big surprise: there is indeed a peak near the center, but then the curve flattens out! Stars are still moving as fast around the center of the galaxy, no matter how far they are! That doesn’t make sense; there’s not enough visible mass to explain that. And that’s where Dark Matter comes in: it’s additional mass (Matter) to explain those rotation curves, but that we forgot to count when we were measuring the brightness of stars simply because, well, it doesn’t shine! (Dark)

Here’s a nice video to explain it more visually:

It might seem like a very arbitrary thing to do – you’re missing some mass? just pretend there’s a new type of matter! And in fact it is. But it turns out that when you do add this concept of Dark Matter to the composition of the Universe, it helps out a lot: it stabilises galaxies, allows for their formation, shapes the evolution of the Universe, etc. And all the different measurements we made are consistent together, so it ends up being a neat solution.

Now for the particle physics perspective: what makes Dark Matter “dark”? Well, to scientists like me, it means that it doesn’t interact with photons, the particles of light – if it emitted photons, it would shine by definition (either in visible light, or infrared, ultraviolet, etc.). And we also know that Dark Matter particles must be pretty stable, otherwise they would decay rapidly and send off high energy particles we could detects (like cosmic rays). All of this is telling us: we’re looking for an unusual particle, that’s quite massive, doesn’t produce photons and interacts at best very weakly with ordinary matter so that it doesn’t decay or only rarely.

This makes producing Dark Matter potentially very difficult, but we’re hoping that by smashing protons together at the Large Hadron Collider at high enough energies, we will eventually reach a point where we can produce these new particles and “force” them to decay inside our detector. So far we haven’t been successful, but we’re still actively looking! All I’ve told you so far is very generic (which I find actually quite elegant, because it’s a very simple solution), but there are many, many different theories about what Dark Matter might look like exactly, and we need to explore them further 🙂