About 20 years ago astronomers became convinced that distant galaxies were moving in ways that made no sense, given the laws of gravity and the fabric of celestial objects visible in the sky. Gradually they were forced to conclude that the universe is not as empty as it appears, that in fact it must be dominated by some dark kind of matter. Although no one knew what the stuff is made of or how it works, scientists could see from its effects that it is out there. The quest to understand dark matter (and more recently, dark energy) meant revising or replacing theories, but it reenergized astrophysics and cosmology. A similar revelation is now unfolding in molecular genetics. This year biologists celebrated the 50th anniversary of the discovery of the double helix, and the Human Genome Project announced its completion of a "final draft" of the DNA sequence for Homo sapiens. Scientists have clearly mastered DNA in the lab. Yet as they compare the DNA of distantly related species and look more closely at how chromosomes function in living cells, they are increasingly noticing effects that current theories cannot explain.
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