Astronomers are very sure that dark matter exists, but they’re not sure at all what it’s made of.
The problem is that it isn’t just dark, it’s invisible. As far as we know, dark matter doesn’t emit light, absorb light, reflect light, refract light, scatter light, diffract light, or really have anything to do with light at all. This makes it hard to study. We know that dark matter exists, however, through its gravitational effects. Even though it’s invisible, it still has mass, and so the dark matter in our universe (which, by the way, makes up 85% of all the mass in the cosmos) can affect the motions of normal (or light-interacting) matter, like stars and galaxies.
But there could be a way to directly see dark matter, and that depends on what exactly the dark matter is made of. There are a great number of interesting theories as to what could be the particle (or particles!) behind the dark matter, and one of the leading candidates involves a creature known as the sterile neutrino.
Neutrinos have been known for decades, and at one time they were, as a group, thought to be the dark matter. They have a tiny bit of mass, but don’t interact with light, so they might’ve fit the bill. But their fatal flaw is that they’re too hot: they stream throughout the universe as too great a speed, and all that commotion would’ve smoothed out the formation of larger structures in the cosmos.
In other words, if neutrinos were the dark matter, then galaxies couldn’t have formed.
So that’s not going to work, but a hypothetical cousin of the neutrinos could work: the sterile kind. These neutrinos are purely hypothetical. If they exist, they wouldn’t just lack an electric charge, but also the weak nuclear charge, rendering them almost completely invisible. And if they have just the right properties, they could be responsible for the dark matter.
But sterile neutrinos may not stay as sterile neutrinos forever. They can (theoretically) occasionally interact with normal neutrinos, decaying in a shower of high-energy radiation. Specifically, X-rays. Even more specifically, X-rays with an energy of 3.5 thousand electron-volts (3.5 keV for short).
So if sterile neutrinos are the dark matter, and there’s a lot of dark matter floating around the galaxy, then there ought to be a faint X-ray glow to the Milky Way, from all the sterile neutrinos turning themselves into radiation.
Back in 2014, a group of astronomers had thought they found such a signal. It was barely above a detectable threshold, but they claimed a firm observation. Recently, however, other astronomers have looked at the darkest parts of the Milky Way with new and improved instruments, specifically targeting patches that were free from as much contamination as possible.
So far, the new research casts doubts on the 2014 claim, and the existence of sterile neutrinos as the dark matter altogether. All hope isn’t lost, however, and astronomers are still hard at work, searching for that telltale 3.5 keV signature.