A black hole is a place in space where there is a huge amount of mass in a very small area. Because gravity is the same as mass times 1/volume, when you have a lot of mass in a small volume it makes a lot of gravity. Apparently, it makes so much gravity that even light can’t get away from the black hole! That’s why it looks black to us. So black holes happen when the speed you would have to be going to overcome the gravity and get away from the black hole is bigger than the speed of light.
Or think of space and time the way physicists do, as being both part of one thing, spacetime. In that case, a black hole might be a place where spacetime bends in a special way. So when things near the black hole go forward in time, as everything does, they also have to go forward in space. That pulls them deeper into the black hole.
See how she spins faster when she pulls in her arms?
Astronomers think most black holes are about the size of large stars. They probably get started after a supernova when big stars collapse in on themselves. There may also be a lot of teeny-tiny black holes, called micro black holes. But nobody has seen one or proved that they must exist. These micro black holes might have formed during the Big Bang.
Many astronomers think that there are black holes at the center of galaxies of stars, including our galaxy, the Milky Way. These would be extra-big black holes, millions or billions of times the mass of our Sun.
Nobody’s ever seen a black hole, even with a telescope. So the only way we know they exist is that we need them to make calculations about space come out according to the laws of physics. New detectors here on Earth, though, recently picked up tiny vibrations. These vibrations are probably the gravity waves created when two black holes run into each other billions of miles away – like the last ripples from throwing a stone into a pond.
Because stars spin around, black holes made from old stars probably also spin around. Probably these black holes spin very fast, because they are so much smaller than the stars they came from, just as an ice skater or ballet dancer spins faster when she pulls her arms tight in to her sides, because of conservation of momentum.
Learn by doing: throw a pebble into a pond and watch the ripples, then spin around with your arms out and suddenly pull your arms in tight.
More about supernovas
More about gravity