https://www.scientificamerican.com/article/milky-way-dark-matter-signals-in-doubt-after-controversial-new-papers/

We know it’s there, but we don’t know what it is: this invisible stuff is dark matter. Scientists are fairly certain it dominates the cosmos, yet its ingredients are unclear. For a while astrophysicists have been excited by two potential signals of dark matter in space: an unexplained excess of gamma-ray light in the center of the Milky Way and a mysterious spike in x-ray light spotted in some other galaxies and galaxy clusters. The signals have been interpreted as possible evidence of dark matter annihilating itself and decaying into different particles, respectively, but two new papers seem to dampen both hopes. Some say it is time to look for different routes to dark matter. Other researchers, however, maintain that either of these signals could still turn out to be the answer.

The x-ray spike, seen as a bright line of emission at an energy of 3,500 electron volts (3.5 KeV), was first spotted in 2014 and has now been identified in numerous galaxy clusters, as well as in our neighboring galaxy Andromeda. The excitement here stems from the fact that one promising dark matter candidate, a brand of particle known as a sterile neutrino, is expected to naturally decay into ordinary matter and produce just this kind of emission line. Recently Benjamin Safdi of the University of Michigan and his colleagues decided to look for this line in our own galaxy by analyzing a massive amount of data from the X-ray Multi-Mirror Mission (XMM-Newton) telescope. The team took images of various objects gathered for other purposes and blocked them out to instead look in the dark “empty space” off to the side for the 3.5-KeV light. After amassing what amounts to a total exposure time of about a year, the researchers saw no sign of the spike. Their findings came out today in Science. “Unfortunately, we saw nothing,” Safdi says, “and the result is that the dark matter interpretation of this line is ruled out by many orders of magnitude.”