5 dark matter candidates, illustrated

For nearly 150 years, scientists have known something is lurking in shadows of the universe. Something we can’t see, yet makes up most of the matter in the universe. This dark matter could account for over a quarter of all the stuff in the universe, while its cousin dark energy makes up nearly 70% of the universe.

Despite its prevalence, scientists have yet to observe dark matter directly. It does not seem to interact with electromagnetic forces, which makes it very difficult to detect. That doesn’t mean, however, that scientists don’t have theories about how to explain this invisible mystery. 
 

Over the decades, many candidates for dark matter have been proposed, from microscopic black holes to gigantic stars that don’t emit light. While some have been ruled out, others remain at the forefront of ongoing research.

In general, scientists have three basic criteria for what makes a great dark matter candidate. First, they must make sense with what we know about the big bang and history of the universe. Second, they should be discrete or individual enough to be well-defined on their own. And third, the theory behind them must make sense beyond simply explaining dark matter. 
 

Here are five of our favourite dark matter candidates from history, and how they work with those criteria.

1. WIMPs 

Weakly Interacting Massive Particles (WIMPs) are a class of theoretical elementary particles that interact with gravity and the weak nuclear force, but not electromagnetism or the strong nuclear force. A WIMP would need to be big enough to move slowly and only interact with the two weakest forces, making them undetectable by electromagnetic means. Despite decades of searching, WIMPs remain elusive. 

2. MACHOs 

Massive Compact Halo Objects (MACHOs) are objects on the furthest outer reaches of galaxies, so far that they are past the visible galaxy but still caught in its gravitational system. These could be planets not orbiting stars, dim brown dwarfs, black holes, and neutron stars. 

While MACHOs most certainly exist, they can’t account for enough dark matter to be the sole cause. They also struggle with what we know about the history of the universe, mostly because the big bang couldn’t have produced enough matter to make that many MACHOs. 

3. Primordial black holes 

Primordial black holes are the universe’s first black holes, formed not long after the big bang. Rather than forming from supernova compression, like more modern stellar black holes, primordial black holes would come from subatomic particles packed tightly together to the point of gravitational collapse. 

Primordial black holes could be microscopic in size yet still incredibly dense, which could be useful for explaining dark matter. But doubt has been cast on primordial black holes as a significant source of dark matter. Measurements from Voyager 1, taken beyond the far reaches of the solar system, suggest that they aren’t widespread enough to contribute to dark matter. Then, in 2019, observations of the Andromeda galaxy suggested that tiny black holes don’t even exist. 

So while primordial black holes are consistent with the history of the universe, the evidence isn’t sufficient to be a leading contender for dark matter. 

4. Axions 

Axions are a perfect candidate for dark matter. If they exist. These theoretical particles are low in mass, uncharged, long-lived, and slow-moving. First proposed in the 1970s to solve a completely different problem called the strong CP problem, axions make sense with the history of the cosmos, are individual enough to be a candidate, and could solve many problems in physics, not just dark matter. So far, however, there is no sign of them. Efforts are ongoing to design new techniques to detect these elusive particles. 

5. Self-interacting dark matter 

Most dark matter candidates don’t interact with ordinary matter. If they did, we would be able to detect them. Self-Interacting Dark Matter (SIDM), however, interacts with itself. The idea popped up in 2000 and has recently been used to pose solutions to two major problems in physics. The first, the “last parsec problem,” could use SIDM to explain why supermassive black holes merge together instead of getting stuck in a binary dance around each other. It could also explain the core-cusp problem, which sees a discrepancy between the density of dark matter as you get closer to the centre of a galaxy. 

And more! 

There are, of course, plenty of other dark matter candidates out there. From ultralight bosons to right-handed neutrinos and more, the search for dark matter goes on. It remains one of the most interesting unsolved puzzles in physics.