It is unknown what exactly happens to the mass inside a black hole. It is possible that a gravitational singularity forms at the center - a point of zero size and infinite density - or perhaps quantum effects prevent that happening. However, in either case, the event horizon is at some distance from any such point, so the force of gravity is a little weaker at the event horizon (although still exceedingly strong). That means that regardless of the interior conditions, our current understandings of physics can be used to predict what may happen in the region of the event horizon. In 1974, British physicist Stephen Hawking used quantum field theory in curved spacetime to show that in theory, the force of gravity at the event horizon was strong enough to cause energy to "leak" into the wider universe within a tiny distance of the event horizon. In effect this energy acted as if the black hole itself was slowly evaporating (although it actually came from outside it).