Shedding light in our galaxy's dark matter.

There's more to the cosmos than meets the eye. To keep a rapidly rotating galaxy from flying apart, to explain how fast-moving galaxies in a cluster stick together, and to account for the evolution of large-scale structure in the universe, astronomers have been forced to hypothesize that the universe harbors hidden gravitational glue. The seemingly empty blackness of space must abound with vast amounts of dark matter-faint or invisible material adding up to at least 10 times as much mass as the visible stars and galaxies have.

Researchers have hotly debated whether dark matter consists of ordinary matter, unknown elementary particles or other exotic material, or a combination of both. Astrophysicists now report that at least half the missing glue in our own galaxy resides in ordinary dead stars called white dwarfs.

Because dark matter can't be seen, David P. Bennett of the Lawrence Livermore (Calif.) National Laboratory and an international team of colleagues relied on an indirect technique to detect its presence. Any massive object, visible or not, can serve as a gravitational lens, bending and brightening light emitted by a body that happens to pass directly behind it.

To reach Earth, light from stars in the nearby Large Magellanic Cloud must pass through the Milky Way's halo-a region of dark matter that extends far beyond our galaxy's visible outline. By monitoring changes in the brightness of 9 million of these stars nightly, Bennett's team hoped to detect the gravitational lensing that clumps of dark matter would cause.

In analyzing the first 2 years' worth of data from their continuing search, the researchers found seven lensing events that they ascribe to assemblages of dark matter known as Massive Compact Halo Objects (MACHOs). Interpreting these results proved difficult, because the team can measure each event only by a single number that depends on three unknowns: the mass, speed, and location of the MACHO.

By combining the seven events and making standard assumptions about the distribution of dark matter within the halo, the researchers conclude that MACHOs most likely have a mass between one-tenth and one solar mass. This places the unseen matter in the domain of white dwarfs, which represent the burned-out remains of stars like the sun.

Calling the result "surprising," cosmologist David N. Spergel of Princeton University says many astronomers had expected that the team would find about 30 shorter-duration lensing events or virtually none. The larger number would arise if brown dwarfs, objects not quite massive enough to be stars, make up most of the dark matter in our galaxy. The absence of lensing events would indicate that the dark matter consists of uniformly scattered exotic material. "It's a very exciting result, but with only seven events, it's not a definitive one," Spergel adds.

Rosemary F.G. Wyse of Johns Hopkins University in Baltimore cites a possible discrepancy between the results and the chemical evolution of our galaxy. In becoming white dwarfs, stars shed a significant amount of helium. However, the Milky Way doesn't have the high helium concentration expected from a large population of white dwarfs. Joseph I. Silk of the University of California, Berkeley, notes two loopholes: The helium might have blown completely out of the galaxy, or the dwarfs, which presumably formed billions of years ago, might have unusual properties that allowed them to retain most of their helium allotment.

He and Spergel agree that even if most of the Milky Way's dark matter is ordinary stuff, prevailing theories about the evolution of the universe and the abundance of elements forged in the Big Bang all but demand that the cosmos at large harbor huge amounts of mystery material-dark matter that bears no resemblance to ordinary atoms.