Season 17, Episode 16: Dark Matter Candidates Explained

Hey StarTalkers! Season 17, episode 16 had Neil and Chuck discussed the “dark” universe with Professor Katherine Freese.

A fan question called on Professor Freese to run-through the leading candidates for the unknown source of gravity known as dark matter:

Dark Universe Decoded with Katherine Freese - StarTalk Radio

(from 33:30)

She names three key candidates: WIMPs, axions and primordial black holes. But what are they? And why do we think they could make up dark matter?

WIMPs

These Weakly-Interacting Massive Particles are probably the most well-known dark matter candidate.

The “weakly-interacting” part isn’t an insult. They literally interact with ordinary matter through the weak force. A 2024 paper explains that in the energetic, early universe, weakly-interacting particles were created thermally and frequently interacted with normal matter.

But as the universe expanded and cooled, the interactions stopped and left behind a “relic” of now basically inert particles. This relic would be pretty close to the amount of dark matter we see today.

Many particles could be WIMPs, including neutrinos, supersymmetric particles (like the neutralino) and the inert Higgs doublet. Beyond the fact that some dark matter is probably neutrinos, we aren’t too sure.

Axions

Understanding axions fully would require a whole post, but the basics are easy enough to get. In order to solve an outstanding problem in particle physics, two physicists proposed a new symmetry, which can be spontaneously broken. Just as the Higgs Boson is the particle associated with the breaking of electroweak symmetry, the axion is the particle associated with the breaking of this symmetry.

The major issue with this is that axions barely have mass, so we’d need a lot of them to account for dark matter.

Primordial Black Holes

In the early universe, it may have been possible for much lighter black holes to form than today. One with the mass of Earth, for instance, would be about the size of an American dime. These tiny, primordial black holes could be zipping around the universe today and make up a substantial portion of dark matter.

There are limitations on their size – Hawking radiation would have “evaporated” tiny ones already, for instance – but LIGO’s detection of gravitational waves from merging black holes has reignited interest.