Dark matter is believed to make up roughly 85 percent of the universe’s mass and yet, we know almost nothing about it. Although scientists have still not detected this mysterious substance directly, they infer its presence from the effect it has on stars and galaxies.

Some theories about the composition of dark matter suggest that it consists of Weakly Interacting Massive Particles (WIMPs) that release high-frequency gamma rays when they annihilate each other. If this is true, this should lead to a gamma-ray excess — one that can’t be explained as originating from any known phenomenon.

A team of astronomers using NASA’s Fermi Gamma-ray Space Telescope has now discovered one such signal originating from the center of the Andromeda, or M31, galaxy — a spiral galaxy located about 2.5 million light-years from Earth. The scientists involved in the research believe that the signal, which is similar to the high-energy gamma rays being emitted from the center of the Milky Way, may indicate the presence of dark matter.

“We expect dark matter to accumulate in the innermost regions of the Milky Way and other galaxies, which is why finding such a compact signal is very exciting,” lead researcher Pierrick Martin, an astrophysicist at the National Center for Scientific Research and the Research Institute in Astrophysics and Planetology in Toulouse, France, said in a statement. “M31 will be a key to understanding what this means for both Andromeda and the Milky Way.”

However, dark matter is not the only possible source. The gamma-ray emissions may be coming from a rich concentration of pulsars (rotating neutron stars) in Andromeda’s center. Further observations would be needed to confirm whether the gamma rays are being emitted due to the annihilation of dark matter particle or from pulsars.

Crucially, the detection of similar signals from the Milky Way and the Andromeda galaxy means scientists can compare the two for the purpose of future studies. It may also reveal clues about the formation and evolution of spiral galaxies, given that the Fermi telescope views Andromeda from an outside vantage point — one that impossible to attain in the Milky Way, whose center remains partially obscured.

“Our galaxy is so similar to Andromeda, it really helps us to be able to study it, because we can learn more about our galaxy and its formation,” study co-author Regina Caputo, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in the statement. “It’s like living in a world where there’s no mirrors but you have a twin, and you can see everything physical about the twin.”