The most elusive black holes in the universe aren't the big ones, or the small ones. They're the medium ones — and a team of astronomers has proposed a new method, using ripples in space-time, to hunt for them.
Known black holes come in two general varieties. There are the stellar-mass black holes, which range from a few to a few dozen times the mass of the sun. And then there are the supermassive black holes, which range in mass from a million suns all the way up to 50 billion solar masses.
