What If We Could See Dark Matter? | Unveiled

What If We Could See Dark Matter?

Observable, ordinary matter makes up just 5% of the universe. Everything we can see consists of it; all the stars in the sky, all of the planets, every single human being and all the food you’ve ever eaten. But will we ever be able to observe everything else that’s out there?

This is Unveiled, and today we’re answering the extraordinary question; what if we could see dark matter?

Around 25% of everything in the universe is dark matter, an unknown type of matter with mysterious properties we don’t understand. The other 70% of the universe (that isn’t dark or ordinary matter) is made up of dark energy, which is in many ways even more elusive than dark matter is. The word “dark” in this context simply means they’re both unknown rather than that they’re connected to each other, so we can certainly talk about them in isolation. Dark energy is the substance we think causes cosmic expansion. Dark matter, on the other hand, is speculated to exist because of how we currently understand gravity to work; it’s believed to be there because of various gravitational effects we’ve observed which don’t make sense without it. Overall, while we know roughly what dark matter does and where it probably is, we’re still not sure on exactly what it is.

The gravitational effects we’re talking about are seen most clearly in the formation of galaxies. Galaxies are enormous, and we initially expect that things further out from the galactic centre should orbit at slower and slower speeds. But this has been found not to be the case; distant objects orbit at roughly the same speeds as close ones; at around 250 kilometres per second in the Andromeda galaxy, for example. It was this which gave rise to the theory of dark matter, with the suggestion that it’s this strange substance that helps galaxies to keep their otherwise inexplicable shape.

Handily, we can also plot the placement of dark matter through gravitational lensing, a phenomenon where light from distant galaxies is distorted by the gravitational pull of massive objects in its path. In some cases, although we can observe light being warped, we aren’t able to detect the object responsible - so dark matter is to blame. It’s either that, or almost everything else we know about gravity - starting with Einstein’s theory of relativity - is wrong. Today, the existence of dark matter is viewed as more likely.

But all of our observations, suppositions and theories still aren’t enough to prove that dark matter definitely exists. There are, though, many experiments running around the world trying to do just that - by directly detecting it. SNOLAB is an operation deep inside a nickel mine in Ontario, Canada, which has a handful of dark matter detectors within. One detector, known as DAMIC, is trying to take images of various particles to hopefully one day find a new one that could be deemed “the first dark matter particle”. In the same facility, DEAP-3600 is trying to detect dark matter particles as they pass through argon. Elsewhere, other experiments include XENON1T in Italy, which is trying to discover “WIMPs” – or, “weakly interacting massive particles” – by observing whether xenon gas will light up when one passes through it. As of late 2019, however, neither these nor any other detectors around the world have been successful in their search, though physicists stress that this isn’t necessarily a bad thing. We still don’t know what dark matter particles look like or how they behave, but all of these duds at least show us what they don’t look like and how they don’t behave. By process of elimination and by tweaking the experiments we’ve set up so far, we may one day be able to “see” the particles we so ardently suspect are there.

To take a different approach, we’ve also tried to map dark matter through simulations and artist renditions of outer space. These are again largely based on the perceived behaviour of gravity, especially in the formation of galaxies, but some sims have led some scientists to think that the Milky Way and most other galaxies might have a “dark matter halo”. If true, it’s this halo that keeps a galaxy so rigidly structured, with the dark matter inside the halo gravitationally bound together just like normal matter is. The Milky Way’s proposed halo is thought to be spherical and to comfortably encompass our entire galaxy - being much, much larger than the Milky Way itself. Interestingly, however, according to a November 2019 study published in “Nature Astronomy”, there are some dwarf galaxies which don’t appear to have this halo, with at least nineteen structures out there seeming not to harbour dark matter in the same way - they behave as if all the mass they have is the regular matter we can see. These seeming anomalies serve to throw the debate wide open, though, with some arguing that the galaxies without simply “lost” their dark matter somehow over time… while others see the absence of dark matter in these galaxies as evidence that dark matter doesn’t exist at all. Currently, we’ve no way to tell which side is right, but the people who do believe in dark matter are still in the majority.

Since we can’t see any subatomic particles with the naked eye, were those that make up dark matter to be something that we understood more, then despite the fact that we’d be surrounded by it all the time… we still might not be able to “see” a lot of it for ourselves. We can’t see the individual particles in the air, for example, but we know that they’re there because of our ability to breathe. This means that the predicted dark matter halo, where the concentration of dark matter is thought to be especially high, would be the most visible source of dark matter for ordinary people. We can already see certain sprawling, dazzling parts of the Milky Way in the night sky, so if the halo were suddenly visible it would simply become another of its most prominent features.

But would dark matter still be dark matter in this scenario? Since the reason we’re so interested in it is because it’s completely unknown and strange, if we could see it, we’d be able to study and understand its properties with far more ease. If we could look into the sky and see the dark matter halo, we would understand with close to certainty that that halo is how the galaxy keeps its shape. Or, if we could see dark matter but the halos still didn’t exist, we’d know that it really was our understanding of gravity that would need another going over. In either case, dark matter that’s visible would need redefining; the mystery would be lifted, and the “darkness” would have disappeared.

And yet, the hypothetical unveiling of dark matter to the world could bring with it countless other problems to solve as well. Some, less mainstream theories suggest that if there’s dark matter there, then it could potentially make up whole structures, perhaps planets, maybe even lifeforms all of its own. In this way, there could be an entire “shadow universe” hidden in plain sight; beyond our comprehension at the moment, but perhaps not if we ever were to crack dark matter.

It sounds farfetched, but it might not be as ridiculous as it seems. Almost all observable matter is already one specific type; baryonic matter, consisting of baryons - usually protons and neutrons. But non-baryonic matter definitely does exist… with electrons, for example, technically being non-baryonic particles called leptons. We’ve already discovered other types of lepton particle in the past, like taus and muons, but also neutrinos, which were once as elusive as dark matter is today. It took more than twenty-five years after neutrinos were first predicted for them to actually be discovered, so could our understanding of dark matter be following a similar path? Given how little we currently know about it, perhaps it isn’t hard to imagine that “dark matter” could contain all kinds of things… So, it arguably could populate a whole periodic table all of its own, accounting for entire, secret civilizations that we might someday be able to communicate with.

It’s clearly a long way between now and then, but visible dark matter would see one of the greatest questions in modern physics finally answered. We would have an unprecedented understanding of how the universe and everything - truly everything - in it forms and functions. And that’s what would happen if we could see dark matter.