Well, they used to think that Black Holes didn't let anything escape, and therefore would exist infinitely. But Stephen Hawking discovered that they "evaporate" because of Hawking Radiation. You see, virtual pairs of particles are being created all over the universe, one with a positive mass, and one with a negative mass. They usually annihilate on each other before they have a chance to do anything, but for the ones being created on the edge of the event horizon of a black hole, they don't. The gravity is so strong there that the particle with negative mass gets sucked in, reducing the black hole's mass, whilst the positive mass particle escapes. Eventually the mass will get so low that it will esplode.