Our galaxy may be surrounded by a swarm of invisible companions made of dark matter.
After the big bang, dark matter should have formed a halo which then attracted ordinary gas to form stars and galaxies. When Jürg Diemand of the University of California, Santa Cruz, and his team modelled this process, they found that there should be at least 10,000 sub-haloes of dark matter within the halo of the Milky Way. Of these some 120 should have attracted some gas of their own and become dwarf galaxies.
Only 15 dwarf companions of the Milky Way have been found. If the remainder have not become visible dwarf galaxies, they could still be detected by gamma-rays given off as the dark matter particles collide.