Two scientific teams reported yesterday that they had independently observed what could be the first evidence that some of the invisible, or dark, matter making up much of the mass of the universe exists in the form of stillborn or extremely dim stars at the edges of galaxies.
Such objects, known as Massive Compact Halo Objects, or MACHOs, have been hypothesized for years as likely candidates for dark matter. The acronym was chosen to contrast with theories invoking exotic subatomic particles, as yet undiscovered, bearing the name WIMPs, for Weakly Interacting Massive Particles.
If the new findings of MACHOs on the fringes of the Milky Way galaxy are confirmed by further sightings, astrophysicists said, this would be the first observational breakthrough in astronomy's concerted search for the mysterious dark matter.
It would be the first identification of the unseen matter that causes galaxies to weigh 10 times as much as they appear to in visible light or other detectable radiations. The existence of these greater masses has been inferred by their gravitational effects on the shape and motion of the galaxies, but their form has eluded detection.
The discoveries were announced in cautiously worded statements at two scientific conferences in Italy. U.S. and Australian scientists said they hurried to report their results when they learned that a French team planned to make public the results of their own similar but independent observations.
The U.S.-Australian team, led by Dr. Charles Alcock of Lawrence Livermore Laboratory in California, reported that in monitoring 3.3 million stars for a year, they had detected one remarkable event that could reveal the existence of dark matter in the form of MACHOs.
A star in the Large Magellanic Cloud, a neighboring galaxy, began to grow brighter in late January. By March 11, it appeared to be seven times brighter than normal, and since then, it has diminished to its previous brightness.
After ruling out atmospheric distortion and other possibilities, the scientists concluded that they had probably witnessed a "microlensing" event. Microlensing occurs when a massive object crosses the line of sight between a telescope on Earth and a star in a distant galaxy.
The gravity of the object acts as a converging or amplifying lens, which makes the star appear brighter for a short time. The event is temporary since the star, the object and the telescope are moving relative to one another.
The French scientists, led by Dr. Michel Spiro of the National Center for Scientific Research at Saclay, near Paris, made a similar observation with a telescope at the European Southern Observatory in Chile.