As a kid you ever play that game Guess Who? If you haven’t, it’s actually kinda fun. You have two players, each with a board in front of them. On the board are a bunch of flip cards with different characters. You have to guess your opponent’s secret identity through a process of elimination. You ask if they’re a kid or an adult, or a boy or a girl, or if they’re wearing glasses or bald. If you ask the right questions, and eliminate the correct possibilities, you’re left with only one remaining option: your opponent’s secret identity.
Whenever I think about our search for dark matter I can’t help but think about that game. We have something going on in the universe. One kind of observation – measuring the amount of hot, glowy, visible matter – isn’t lining up with other kinds of observations, ones that tell us the TOTAL amount of matter in the universe. To explain this discrepancy, we have an exceedingly wide variety of options.
All of those options fit into two broad categories (which I should stress are not mutually exclusive, it’s more than possible for the universe to be exceedingly complicated) and they are that either there is some new ingredient in the universe, some heretofore unknown form of matter, or that our understanding of gravity is fundamentally flawed at galactic scales.
Theorists are no slouches, and as soon as the evidence for something new and exciting appeared in our observations, they started cooking up ideas. And so within each of these categories there is a dizzying array of models, hypotheses, wild guesses, and brilliant-but-wrong insights. Beginning with Vera Rubin’s groundbreaking measurement we’ve been hard at work playing a game of galactic Guess Who. Any idea of what’s causing the discrepancies in the observations has a huge challenge in front of it: it has to explain ALL observations, pass ALL tests, at ALL scales, at ALL times, from galaxy rotation curves to the cosmic microwave background…and more.
For example, one might reasonably assume that maybe the dark matter is just…dim matter, like black holes, you know, something that’s not easy to see. It’s easy enough to fill up a galaxy with extra black holes to account for rotation curves. But how do you get those black holes in the spaces BETWEEN galaxies to account for cluster observations? Or populate the early universe with enough black holes to account for the cosmic microwave background? The cosmos isn’t all that great at forming black holes – first you need stars to live and die to make some – so you can’t account for all the dark matter just through black holes alone. An abundance of black holes may explain one observation, but not all.
Or maybe it’s neutrinos. You know, those ghostly little particles that hardly ever interact with normal matter? At first glance, they’re an ideal dark matter candidate. There’s a lot of them. They have a little bit of mass. They’re really, really hard to see. But in this game of Guess Who, we have to eliminate them. That’s because they’re hot – in cosmology terms, they travel at nearly the speed of light. This means they’re not good for building structures like galaxies. Neutrinos make them too smoothed out. If we were to fill the universe with enough neutrinos to explain the dark matter, then we can’t get galaxies to form. They would just be spread-out blobs of gas. An abundance of neutrinos may explain one observation, but not all.
What about alternative forms of gravity? Perhaps the most famous of these is MOND, for Modified Newtonian Dynamics. Essentially it’s a fix to Newton’s gravity carefully crafted to explain galaxy rotation curves. It just says that at large scales, our usual understanding of mass, force, and acceleration is a little bit off. Yeah it’s not the most rigorous thing in the world but it’s a good start. The trouble with MOND, and all of its related theories, is that while they can be tuned for one particular scenario, they have trouble extending to other scales.
For example, you might be able to nail galaxy rotation curves, but you then have difficulty getting the details of clusters right, especially interacting ones like the famous Bullet Cluster. This is a composite image showing the components of a merger between two galaxy clusters. You can see the individual member galaxies, which have largely sailed past each other unharmed (because there’s a lot of space in a cluster). You can see the hot gas in the middle, which got all tangled up and shocked. And then you can see with gravitational lensing where most of the matter is – and it’s not in either place. Simultaneously explaining images like these, ALONG WITH galaxy rotation curves, ALONG WITH the cosmic microwave background is a task that NO theory of modified gravity has accomplished.
It doesn’t mean that we’re getting gravity 100% right, but to date nobody’s been able to figure out a compelling alterative that explains all observations. Even if these theories are correct, or at least on the road to being correct, the fact that they always come up short in one direction or another means that dark matter is unavoidable. You simply can’t get rid of it.
After half a century of Guess Who, we’re left with very few possibilities. Modifying gravity isn’t cutting it. An abundance of normal-but-otherwise-dim-or-invisible matter isn’t cutting it. The only hypothesis remaining is that dark matter really is a new kind of matter, some mysterious particle currently unknown to physics.
The dark matter has to have a limited set of properties to match all known observations. It has to rarely, if ever, interact with light. It has to rarely, if ever, interact with normal matter. It might be able to interact with itself to some degree, depending on the model. It has to be cold, which means that it has to move slowly compared to the speed of light. And it needs to be made in abundance in the extremely early universe so that it can participate in all of these cosmic shenanigans from the get-go.
For decades cosmologists were sure they were on the right track. They even had a name for their prime candidate, the WIMP: the Weakly Interacting Massive Particle. And so they set up detectors and specialized observatories around the globe to get that definite proof that they were looking for.
