What is a Black Hole?
Scientists believe that some regions of space exert so much gravity that they act like giant vacuum cleaners. Anything that gets too close gets sucked in. That matter is crushed to zero size and therefore infinite density. (Remember, density is an object's weight divided by its size, so as size gets infinitely small, density gets infinitely large.) It disappears forever.
The event horizon, the boundary of these giant vacuums, is formed by the rays of light that can't quite get away from the black hole, but stay forever, hovering on the edge. The gravity is so intense that it tugs at space and time, slowing down time and stretching out space. Not even light, the fastest thing in the universe, can go fast enough to escape this enormous gravity. In 1969, the American physicist John Archibald Wheeler appropriately named these dark, devouring voids "Black Holes"!