Scientists believe that some regions of space exert such powerful gravity that they act like gigantic vacuum cleaners, sucking in any matter that comes too close. That matter--whether it is a comet, a planet, or a cloud of gas - is crushed to infinite density and disappears forever. The gravity is so intense that it tugs at time and space, slowing down time and stretching out space. Not even light can escape the fierce gravitational pull, so they remain black and invisible. The American physicist John Wheeler named these dark, devouring voids black holes. Black holes seem more like science fiction than reality. In fact, the idea of black holes was created in the imaginations of theoretical physicists and has not yet been verified by the conclusive discovery of an actual black hole.

But since 1915, when Albert Einstein developed his theory of relativity, scientists have known that black holes may exist. Einstein's theory proved that if gravity could become strong enough, it would rob light of all its energy, trapping it in the same way in which it traps a planet's atmosphere. For gravity to be that strong, however, its source would have to be an extremely dense object, one with a very large mass compressed into a very small space.

In 1916 the German astronomer Karl Schwarzschild calculated just how compressed a star would have to be for its gravity to trap light. According to Schwarzschild's calculations, a star the size of the sun - 864,950 miles (1,392,000 kilometers) in diameter - would have to shrink to less than 1.9 miles (3 kilometers) wide.

In 1939 the United States physicists J. Robert Oppenheimer and Hartland S. Snyder discovered that it is possible for stars much larger than the sun to become this small. For most of their lives, stars remain a constant size because they contain a balance of forces: heat generated by burning fuel expands the star outward, while the force of gravity pulls it in. Eventually, in billions of years, the star exhausts its nuclear fuel and collapses under its own weight.Oppenheimer and Snyder proved that if the star is more than 3.2 times as massive as the sun, nothing could stop the collapse.

Scientists believe that the Earth's galaxy, the Milky Way, may harbor as many as 100 million black holes left behind by collapsed stars. In 1990 the Very Large Array radiotelescope in New Mexico obtained detailed images of the center of the Milky Way showing tremendous bursts of energy emanating from its center. Some scientists believe this confirms the existence of a black hole with four million times the mass of the sun at the center of the Milky Way.

Scientists using the Hubble Space Telescope in 1992 found promising signs of a black hole with three million times the mass of the sun in a nearby galaxy. Because black holes are invisible, astronomers have been trying to locate them by observing their effects. Matter swirling toward the center of a black hole should emit rapidly pulsating and detectable X-rays. In 1965 astronomers observed intense X-rays radiating from the constellation Cygnus, nearly 10,000 light-years away. In 1971 the world's first X-ray satellite pinpointed the exact location of these X-rays, a massive but invisible object that astronomers have named Cygnus X-1. Cygnus X-1 may prove to be the first identified black hole.