The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

the more that astronomers discovered about the system of “fixed stars”—that the stars aren’t fixed at all but are in motion relative to one another, and that the entire system of unfixed stars, our galaxy, rotates around a common center—the less satisfying was the explanation of inaction at great distances. Einstein made subtle adjustments to Newton’s theory of gravity. And in his 1916 theory of general relativity, he presented calculations on paper that matched the motions in the heavens slightly more accurately than Newton’s. Yet he, too, had to account for a universe that, as was evident in “the small velocities of the stars,” wasn’t collapsing of its own weight. In his 1917 paper “Cosmological Considerations on the General Theory of Relativity,” he inserted a fudge factor in his equation—the Greek symbol lambda, “at present unknown”—to represent whatever it was that was keeping the universe from collapsing. Like Newton, he feigned no hypotheses as to what that something might be. It was just . . . lambda. But then, little more than a decade later, came Hubble’s universe, and with it an elegant and unforeseen solution to the lack-of-collapse conundrum: The reason the universe wasn’t collapsing of its own weight was that it was expanding.