Dark matter remains a mystery of modern cosmology. Something unseen in the universe is exerting gravity, affecting how galaxies spin and ensuring that whole clusters of galaxies don’t drift apart. It’s widely accepted that dark matter exists and that, like normal matter, it has mass. Unlike normal matter, however, it is invisible, or “dark.” Most scientists favor the theory that dark matter is made up of particles like visible matter, but every attempt to directly detect or even conclusively characterize those particles has proven unsuccessful.

But now a startling new theory posits that, rather than comprising new, exotic particles, dark matter is actually made up of black holes. "Studies are providing increasingly sensitive results, slowly shrinking the box of parameters where dark matter particles can hide," said Alexander Kashlinsky, an astrophysicist at NASA’s Goddard Space Flight Center. "The failure to find them has led to renewed interest in studying how well primordial black holes - black holes formed in the universe's first fraction of a second - could work as dark matter."