If you put enough matter in one small place, it creates a black hole, a gravitational field so intense that nothing, not even light, can escape from it!

A black hole is an extremely weird object. Its intense gravitational field changes space and time in fundamental ways. Each black hole has only three measurable properties: mass, electrical charge, and rotational momentum.

Most astronomers now believe that there is a stupendously huge black hole in the inner core of our galaxy, billions of times heavier than the sun. It spins in the center of a rotating whirlpool of hot gas and dust, slowly sucking it in.

Black holes are small in size. A million-solar-mass hole, like that believed to be at the center of some galaxies, would have a radius of just about two million miles (three million kilometers)—only about four times the size of the sun. A black hole with a mass equal to that of the sun would have a two-mile (three-kilometer) radius. (source)

The critical distance at which nothing can escape is called the Event Horizon.

The known closest black hole to Earth is Cygnus X-1, located about 8000 light years away.