The popular image of a black hole is fairly uniform: a huge, massive disk in space that sucks in everything in its vicinity. It turns out, a black hole can be any size—there is no known upper or lower limit. There are many types of black holes. And they aren’t even necessarily black. Black holes are mysterious phenomena with a host of mind-bendingly curious characteristics: from event horizons that separate the universe into “outside” and “inside” to the warping of time itself. Robbert Dijkgraaf, mathematical physicist at the University of Amsterdam and expert on black holes, answers a fundamental question—what exactly is a black hole?