The center of our galaxy may be teeming with dark matter particles

The center of our galaxy is behaving strangely, and maybe it’s due to dark matter. In 2009, observations from the Fermi Gamma-ray Space Telescope revealed a surprisingly high concentration of gamma-rays coming from the center of the Milky Way, a phenomenon called the Galactic Central Gamma-Excess (GCE). Now, simulations suggest that these gamma rays could come from the annihilation of dark matter particles.

The debate over what might cause GCE has raged since it was first observed, with two main explanations emerging over the years. The first is that it could come from an as yet undetected population of pulsars, which are rapidly rotating neutron stars that emit rays strong radiation.

Another explanation is that it could come from weakly interacting massive particles (WIMPs), which have long been prime candidates for inclusion dark matter. These particles would hardly interact at all with particles of ordinary matter, but if two of them happened to collide with each other, they would destroy each other and create a gamma ray burst.

In recent years, the dark matter explanation has fallen out of favor, in part because a direct search for WIMPs has turned up nothing. “Given the lack of direct evidence for the existence of dark matter, despite very sensitive searches, a higher burden of proof is required to interpret dark matter,” he says. Jeff Grube at King’s College in London.

Another reason why this interpretation has been sidelined is that we would expect the dark matter in the center of the galaxy to take on a spherical shape, and the GCE is not spherical, but flattened in the middle. However, new simulations by Josip Svil at Johns Hopkins University in Maryland and his colleagues show that this may not be a problem.

These simulations took into account the history of the Milky Way more detailed than previous GCE surveys. “We know that a few billion years ago our galaxy underwent a merger with a number of smaller galaxies, and they brought their dark matter with them,” says Silk. “The consequence of that history is that no one in their right mind would expect the center of the galaxy to be spherically symmetric.”

The results confirmed this, giving a squashed dark matter distribution that fits the shape of the GCE. This brings the dark matter interpretation back to the table. But the mystery is not yet solved – pulsars are also a viable explanation. “This leaves the situation ambiguous at best,” says Grube.

The gamma-ray observatories we currently have are not powerful enough to distinguish between the two explanations, but we should be able to resolve this with the Cherenkov Telescope Array, which is under construction in the Canary Islands and Chile and is expected to begin observations in 2026.

“There’s a 50 percent chance in some sense that we’ve detected dark matter in enormous amounts, but it’s going to take a new telescope to sort that out,” says Silk. If the GCE due to dark matterit will represent the best chance we’ve ever had to understand this strange and enigmatic thing that holds the cosmos together.