Most scientists now agree that the universe began with the cataclysmic explosion of an extremely dense primordial atom, and that the billions of star-filled galaxies, including the Milky Way, are still rushing outward from the original big bang. The speed of that expansion, astronomers have determined, is decreasingslowed by the gravitational pull of the galaxies upon each other. What cannot be explained, however, is that the calculated mass of the universe's galaxies is only about one-tenth the amount required to produce that rate of deceleration. Where is the missing mass?
Black Holes. Some scientists think that the answer lies in vast clouds of hydrogen and helium gas discovered in intergalactic space. But others, including Cosmologists Alastair G.W. Cameron and James Truran of New York's Yeshiva University, doubt that the total mass of intergalactic gas is sufficient to provide the remaining gravitational pull. Instead, Cameron and Truran suggested at a meeting of the American Physical Society, the missing mass may be hid den away in a completely invisible form: inside so-called "black holes" in space.
First postulated in 1939 by Physicist J. Robert Oppenheimer and Hartland Snyder, one of his graduate students at the University of California at Berkeley, black holes are the theoretical residue of extremely massive stars whose thermonuclear fuel has been exhausted. As the fires go out, the gaseswhich have been supported in the shape of a huge, distended globe by heat and radiation suddenly begin to fall inward toward the star's center of gravity. If the star is massive enough, the imploding gases gather such momentum as they fall that they virtually crush themselves out of existence at the stellar center. Using the formulas of Einstein's general theory of relativity, more recent theorists predict that as the star shrinks toward oblivion, the familiar rules of physics may be violated. Its mass becomes infinitely dense, yet occupies no space. Its gravitational pull becomes so intense that no light or other radiation can escape from it. Thus the star cannot be detected by conventional observations. It becomes a black hole, or as Cameron calls it, a "collapsar." If a star-crossed spaceship ever strayed close enough to such a cosmic abyss, it would be drawn immediately into it and vanish completely from sight.
Although the existence of black holes has never been proved, Cameron and Truran speculate that as much as 90% of the universe's mass may exist in this bizarre form. As they explain their theory, the early universe's mass probably consisted of clusters of huge superstars. These primordial giants, as much as 100 times as massive as the sun, eventually cooled, collapsed and disappeared.