According to general relativity, nothing can escape black holes, so they should only get bigger over time as they swallow up detritus. But in the 1970s, Stephen Hawking pointed out that thanks to a quirk of quantum mechanics, matter actually can escape. Quantum mechanics says pairs of particles constantly pop into existence and immediately self-annihilate. When a pair forms at the event horizon, one can fall into the black hole while the other drifts off instead of disappearing. That particle effectively carries some matter away, and eventually, the black hole will evaporate entirely.