There’s a surplus of gamma rays spewing forth from the center of our galaxy, and astrophysicists say they don’t know where it’s coming from. One thing is clear, though: it could not have been caused by anything we consider ordinary in our galaxy.

What it could be, according to Tracy Slatyer of MIT and her colleagues at Fermilab, Harvard, and MIT, is excess light from colliding dark matter particles. If true, this observation would be the first-ever indirect detection of dark matter in our universe.

Physicists believe that dark matter is made of weakly interacting massive particles (WIMPs), which act simultaneously as matter and anti-matter, annihilating each other when they meet. That collision gives birth to normal matter in the form of high-energy photons of gamma-ray light. Currently, we have some of our best eyes, including the Fermi space telescope, pointed toward the Milky Way’s center—where dark matter should be the densest—to search for the results of these collisions. In sifting through data collected but the Fermi telescope, Slatyer and her colleagues found about 10,000 extra photons in the 5,000-light-year space surrounding the center of the galaxy. It was an unexpected result.