It’s too early to say for certain, but astronomers may have picked up a new clue to the nature of dark matter—invisible cosmic stuff with at least five times the mass of all the visible stars and galaxies combined.

The clue comes in the form of gamma rays, a kind of light the human eye can’t detect, emanating from a newly discovered dwarf galaxy called Reticulum 2. Reticulum 2, which hovers beyond the edge of the Milky Way, about 98,000 light-years from Earth, is fascinating in its own right: No more than a few thousand stars (compared with the Milky Way’s hundred billion or more) embedded in a clump of dark matter, it’s similar to the first tiny galaxies that appeared