We've long known that black holes come in two flavors. There are stellar black holes, which form when a giant star collapses in on itself, compressing the mass of five or more our suns into an infinitely small point. They just float around the universe, sucking up anything that gets in their way.
And then there are supermassive black holes. These are a lot more mysterious, and their mass can be as high as millions or even billions of our suns. They don't go anywhere, but stay at the center of their galaxies like the hubs of a cosmic wheel. But it always seemed strange to astronomers that there were itty-bitty black holes and mega-gigantic black holes, but nothing in between. Until now.