Jack Anderson, age 13, of Sarasota, Fla., for his question:

IS THERE REALLY A BLACK HOLE?

Scientists have been unable to prove the existence of black holes because the holes are invisible. The astronomers have tried to locate black holes by observing the effect of their gravitational pull on other objects.

A black hole is an invisible object that astronomers believe exists in space. It has such a strong gravitational force that nothing can escape from it. It is impossible even for light to escape from a black hole.

Some scientists believe black holes formed when stars collapsed inward from their own weight. This would have caused their density to increase greatly.

Some astronomers believe black holes make up as much as a third of the material in the earth's galaxy, the Milky Way.