Science: The Missing Mass

Most astronomers believe that the universe was born billions of years ago in a so-called "Big Bang"—a violent explosion of a primordial clump of matter. Ever since, fragments of that clump —consisting largely of the islands of stars called galaxies—have been flying apart. But the universe is expanding at a steadily decreasing speed; the outward flight of each galaxy is being slowed by the pull of gravity from the others. If that pull is strong enough, the galaxies will eventually be braked to a halt. Then they will begin falling back to crush...

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