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.