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THE DARK-MATTER PROBLEM: High on the list of concepts that astronomical theorists would hate to lose is cosmic inflation. It sounds nutty, but the universe actually makes a lot more sense if you assume that just after it was born all of space went into overdrive, exploding outward for the briefest fraction of a second. Inflation explains, among other things, such mysteries as why the universe looks pretty much the same in all directions and how a peanut-butter-smooth distribution of matter in the young cosmos evolved into today's lumpy distribution, with clusters of galaxies surrounded by empty space.

Inflation theory doesn't just explain things; it makes predictions. Chief among them: the blackness of space is only seemingly empty. In fact, it probably abounds with vast amounts of matter-matter that cannot be directly detected because it doesn't shine. If this theory is correct, then there must be precisely enough of this dark matter so that gravity will forever slow the expansion of the universe without ever quite stopping it, balancing space on a gravitational knife edge between eternal growth and eventual collapse.

Dark matter is more than merely theoretical. The first hint that the cosmos contains more than meets the eye came back in the 1930s, when Caltech astronomer Fritz Zwicky pointed his telescope at the Coma cluster of galaxies and realized that it shouldn't exist. Individual galaxies in the cluster were orbiting each other so fast that they should long since have flown out into deep space-unless gravity from some unseen matter was keeping them together.

Nobody took Zwicky too seriously; the idea was crazy, first of all, and besides, the measurements of orbital speeds were difficult to make and prone to error. Nor did anybody take Vera Rubin seriously when in 1970 she and a colleague at the Carnegie Institution of Washington discovered that some galaxies were rotating too fast on their own axes-again, evidence of extra gravity from unseen matter.

Not until a little more than a decade ago was dark matter finally accepted as a huge problem rather than a nagging anomaly. Observation after observation showed that galaxies moved as if they were embedded in clouds of invisible matter containing 10 times as much mass as was accounted for by visible gas and stars. Clusters of galaxies behaved as if there was 30 times as much dark matter as visible matter exerting its gravitational pull. To satisfy inflation theory, the ratio would have to be even greater: 100 times as much dark matter as visible.

Leaving aside theory, the challenge of identifying and understanding the stuff that makes up most of the universe has become one of the most irresistible-and frustrating-quests in science. For more than a decade, the campaign has proceeded on two fronts: attempts to directly observe the missing matter, and attempts to identify it via computer simulations. Those who do the latter assume that dark matter is made of a given particle or substance, then create a computer model of the cosmos based on that assumption, let it evolve in cyberspace, and see if the result looks like the real universe.

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