Black holes are dead stars that have collapsed
under the gravitational attraction of their own mass. The matter density at the
center of a black hole is extremely high. Some say it is infinite and the
center of a black hole is a singularity. The density is so high that a horizon exists
outside of the black hole where the escape velocity exceeds the speed of light.
Nothing that falls through the horizon can escape, even light. Thus, a black
hole is black because nothing inside can
be seen from outside the horizon.
