Since dark matter does not emit any kind of light, its presence can be inferred only by its gravity. Gravity is what the ROSAT (Roentgen Satellite) detected last April when it snapped X-ray pictures of three galaxies, 150 million light-years away, beyond the constellation Cepheus. This triplet is veiled in a cloud of gas 1.3 million light-years across and as massive as 500 billion suns. The gravity required to contain the cloud is 12 to 25 times greater than that in the galaxies themselves, astronomers from NASA, the University of Maryland and Arizona State University announced at a meeting of the American Astronomical Society. To get so much gravity, the three galaxies must be awash in dark matter. “The discovery,” says David Davis of the University of Maryland and NASA, “suggests that the universe may one day halt its expansion.”
Though theorists sniffed that they knew all along there were gobs of dark matter out there, in fact it was the first time the amount in galaxies had been measured so accurately. “It’s awfully nice to have a direct result showing the amount of dark matter in one of these regions,” says astronomer Lawrence Krauss of Yale University. The amount was right on the money: measurements of bigger clusters of galaxies had found only two or three times as much dark matter as regular stuff, not the 10 to 20 times that theory predicts and the ROSAT found. If other neighborhoods harbor similar amounts, cosmology’s trendiest theory of creation may prove right: it’s the idea that after the big bang 12 billion years ago, the infant universe inflated wildly to some 10 trillion trillion times its original size in about a sextillionth of a picosecond. The inflation theory also says that the cosmos is precisely balanced between eternal expansion and eventual collapse. One day, the world will reach perfect equilibrium. The stars will burn out and matter will evaporate into a whisper of pure energy. Born in a bang, it would go out in a whimper.
The ROSAT weighed only the dark matter; it didn’t identify it. Bad enough that 99 percent of the universe is invisible; worse, 90 percent of it might consist not of garden-variety protons and neutrons (as in people and planets) but particles never seen on Earth. Why? Because so many protons and neutrons would have produced much more hydrogen and other light elements than astronomers find out there. But even before physicists figure out what the exotic matter is, they’ve gone Copernicus one better: not only are we not at the center of the universe, but we’re not even made of the same stuff.
