Black holes, though immensely powerful, can slowly evaporate via Hawking radiation.
Think of a black hole as an extremely deep funnel: anything falling too close to the edge can never come back out. Hawking discovered, though, that new particles keep appearing at the funnel's edge, straight out of empty space. Given enough time, the black hole loses energy and, in theory, disappears completely.
Why it matters The radiation Hawking discovered is still a key test for any future theory of quantum gravity.