WIMPS vs. Axions: What’s Darkish Matter?

Dark matter rules every galaxy. But what exactly is it? Astronomers believe it is a new, exotic particle. You may have heard of some terms like WIMPs or axions. Let’s examine what these terms actually mean.

First off, there are a couple of things we know about dark matter. Astronomers believe that dark matter is a type of particle that was previously unknown to physics. Whatever it is, it makes up about 80% of the mass of the universe. It hardly interacts with light, if at all. It hardly interacts with normal matter, if at all. It hardly interacts with itself, if at all. We also know that it is “cold”, which means that the individual particles do not have very high speeds.

It basically just sits there and gravitates. But this gravity is essential: it holds galaxies together and forms the framework 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 is less of a name than a collective category. In this case, “weakly interacting” means “interacting through the weak nuclear force” (although this interaction is also literally weak). WIMPs would be a novel type of particle that only talks about the weak force with normal matter, which would explain why we rarely see them. In this scenario, WIMPs flood the universe – and might even travel through you right now, though without their gravity you would never know.

The axion, on the other hand, is another hypothetical particle (or actually a category of possible particles) that was motivated by theoretical studies of various laws of symmetry in the universe. It just turned out that if this hypothetical particle existed in sufficient numbers, it would function exactly as we know dark matter should.

Beyond WIMPs and axions, theoretical physicists and cosmologists have come up with all sorts of more complicated possibilities. Perhaps dark matter interacts with itself to some extent. Perhaps there are several types of dark matter particles. Perhaps new forces in physics are at play. Perhaps axions can clump together in strange ways. Maybe this, maybe that.

Yes, dark matter is a great mystery. We know that something unusual is going on in the universe, but we don’t know exactly what. But we know that solving the mystery of dark matter will illuminate a whole new universe of physics.

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