Science & Technology

6 Things Dark Matter Might Be, According to Scientists

When it comes to what makes up the universe, you and everything you know — spatulas, Segways, supernovas, et cetera — are in the minority. Only five percent of the universe is made up of "normal" matter, or the stuff we can see and identify. A much larger portion is comprised of dark matter: a mysterious substance that we can only detect by its gravitational force. Everything in the universe we know about can be broken down into quantum particles, and (most) scientists believe that dark matter is no different. They just need to find the dark matter particle. Here are six candidates that have been nominated for that role — some of them more convincingly than others.

WIMP

A fraction of a second after the Big Bang, the universe was white-hot chaos: New particles and their antiparticles were being created and destroyed constantly. As the universe expanded and cooled, new particles stopped popping into existence and many of the existing particles annihilated with their antiparticle or decayed into lighter, more stable particles.

If the very lightest particle was stable and neutral, it would have survived the chaos and would fill the universe to this day. That's the idea behind the weakly interacting massive particle, or WIMP. The "weak" isn't a dig — it specifically refers to the weak force, which is one of the four fundamental forces in the universe, alongside the strong force, the electromagnetic force, and gravity. This particle would only interact with other particles via gravity and the weak force — not the electromagnetic force, which is why we wouldn't be able to see it. If WIMPs exist, models suggest that they must be five times more plentiful than regular matter, which lines up with how much dark matter scientists think is out there. But every experiment we've run to detect this theoretical particle has failed, and by this point, most researchers have ruled it out.

WIMPzilla

It's possible, however, that instead of only the lightest, stablest particles hanging around, some super-heavy particles that were unstable, but just took a really long time to annihilate or decay (as in, longer than the life of the universe) hung around instead. This theoretical particle would be 10 billion times the mass of a WIMP and is therefore known as a WIMPzilla. While WIMPs have been pretty much impossible to detect, WIMPzillas might be exerting their influence on our planet at this very moment in the form of cosmic rays — and scientists are hard at work studying these high-energy beams to see if they can find this candidate particle.

MACHO

In a contrasting acronym that's too convenient to be accidental, the MACHO, or massive astrophysical compact halo object, is basically the opposite of the WIMP. A MACHO could be anything huge and dark made from normal matter, like non-radiating neutron stars, white dwarfs, or even planets. The idea here is that these massive objects would be too dark to be detected by our telescopes, yet would still exert a gravitational pull on the matter around them that would make it look like there was some invisible, dark-matter-esque force out there in the universe. But just like WIMPs, studies have tested predictions about the influence of MACHOs on the universe and they've come up short.

Axion

WIMPs and MACHOs might exist at two ends of a spectrum, but the axion is in a place all its own. This slow-moving particle is lighter than a WIMP and interacts weakly with other matter, but can also decay into photons — and we might be able to detect that decay. What's more, if axions are dark matter, they could only exist in a very specific mass range, and that makes it even easier to design experiments to test for them. Supernovas and stars like our sun should produce these particles, and scientists are using these astronomical objects to try and detect them.

Gravitino

The gravitino is a theoretical particle that's supposed to be the partner of another theoretical partner, which, in the grand scheme of things, makes it super-duper theoretical. This particle is predicted by theories that combine general relativity with a principle called supersymmetry, which is a scientifically popular idea that essentially makes the equations for force and the equations for matter identical. Supersymmetry says that all boson particles like photons (the particle for light) and gravitons (the theoretical particle for gravity) have a "superpartner" — in this example, the photino and the gravitino — that differs from that particle in a predictable way. In some models, this gravitino could account for dark matter.

Dark Monopoles

There's also a theory that says that in addition to dark matter, there must also be a dark form of electromagnetism that would include other dark particles, like dark photons and dark electrons. In a recent paper, two researchers at the University of California, Davis propose a new particle that could interact with these dark particles: a dark magnetic monopole. A monopole is a theoretical particle that acts like one end of a magnet, and the team says that a dark monopole could be a new candidate for dark matter. Even better, it would be detectable; if dark monopoles interact with dark electrons, they might mess with any regular electrons they encounter, too. We don't yet have the technology to detect such a small interaction, but we may someday.

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Love dark matter? Then you should check out "Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe" by physicist Lisa Randall. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer July 9, 2019

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