WIMPS vs. Axions: What is dark matter?

Dark matter rules every galaxy. But what exactly is it? Astronomers believe it to be some kind of new, exotic particle. You may have heard some terms tossed around, like WIMPs or axions. Let’s explore what those terms actually mean.

First off, there’s a few things that we know about dark matter. Astronomers believe that dark matter is some kind of particle, previously unknown to physics. Whatever it is, it makes up about 80% of the mass of the universe. It barely interacts with light, if at all. It barely interacts with normal matter, if at all. It barely interacts with itself, if at all. We also know that it’s “cold”, which means that the individual particles don’t have very high velocities.

It basically just sits there and gravitates. But that gravity is essential: it keeps galaxies glued together and provides the scaffolding for the entire large-scale structure of the universe.

One of the earliest candidates for the dark matter particle are the WIMPs, for weakly-interacting massive particles. It’s not so much a name as a catch-all category. In this case, “weakly-interacting” means “interacts via the weak nuclear force” (although that interaction is also literally weak). WIMPs would be a new kind of particle that only talk to normal matter via the weak force, which would explain why we only rarely see it. In this scenario, WIMPs flood the universe – and might even be traveling through you right now, though you would never know except for their gravity.

The axion, on the other hand, is another hypothetical particle (or really, category of possible particles) that was motivated by theoretical explorations of various symmetry laws in the universe. It just so turned out that this hypothetical particle, if it existed in sufficient number, would operate exactly like we know the dark matter should.

Beyond WIMPs and axions, theoretical physicists and cosmologists have come up with all sorts of more complicated possibilities. Maybe the dark matter interacts with itself to some degree. Maybe there are multiple species of dark matter particles. Maybe new forces of physics are involved. Maybe axions can clump together in strange ways. Maybe this, maybe that.

Yes, dark matter is a big mystery. We know that something funky is going on with the universe, but we’re not exactly sure what. But we do know that cracking the dark matter mystery will illuminate a whole new universe of physics.