What's the Matter?; May 2000; Scientific American Magazine; by Musser; 1 Page(s)
Dark matter isn't terribly interesting stuff. Its identity may be a mystery, but whatever the material is, it must be deadly dull. It doesn't give off light or cast shadows or cohere into stars; it doesn't do much at all, except exert a brute gravitational pull evident only on cosmic scales. Or so scientists thought. Over the past six months they have wrestled with a radical idea: maybe dark matter leads a richer inner life than it seems.
For most of the seven decades since astronomers first suspected the existence of dark matter, it took all their ingenuity just to prove it. The familiar view of galaxies as big bundles of stars is now passe. Galaxies are really just giant balls, or "halos," of dark matter, with some stars sprinkled in. But what are the bodies unseen? One by one, the possibilities have faded away. Two leading searches-the MACHO survey, which ended in January after seven years, and the ongoing EROS survey-have found too few substellar objects, such as planets or brown dwarfs. Other observers recently glimpsed faint white dwarf stars in the halo, but there can't be too many of them, or else the by-products of their formation would litter interstellar space. In a paper published earlier this year Katherine Freese of the University of Michigan all but gave up the hunt: "Most of the dark matter in the galactic halo must be nonbaryonic."