Among the many puzzles that plague cosmologists, perhaps the most vexing is this: What will be the universe's ultimate fate? Will it continue to expand forever, finally stabilize, or contract and end in a cataclysmic fireball? The answer depends on the amount of matter, and thus gravitational attraction, in the universe. Not enough, and the universe will expand without end. Too much, and the world will end in what some astrophysicists call the Big Crunch. Unfortunately, the amount of matter that is now visible isn't nearly enough to account for the behaviors astronomers have observed, such as the velocity of rotating galaxies. Calculations range wildly, but as much as 90 percent of the required mass may be "missing." Many bizarre particles have been theorized as supplying this missing mass, but none have been detected.
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