ASTRONOMY is a little closer to solving a basic cosmic riddle now that some ``invisible'' dark matter may have revealed itself. The riddle asks what the material universe is made of when it acts gravitationally as though most of its mass is in a form that astronomers have been unable to detect.
Astrophysicists have speculated that the universe may be rich in certain kinds of elementary particles that interact so weakly they are hard to observe. Alternatively, they have suggested that at least some of the unseen dark matter may consist of massive compact bodies the size of Jupiter or larger. They could be wandering planets, burned out stars, or would-be stars that don't have quite enough mass to ignite a stellar fire. They call them massive compact halo objects (MACHOs) because they expect the objects to lie mainly in the outer halo regions of galaxies.
Last week, two different research teams reported that they have detected what looks like the ``signature'' of MACHOs in our own Milky Way galaxy. If further research confirms this identification, it will be the first direct evidence of what some of the dark matter is made of.
This ``signature'' is an effect called gravitational lensing. If a dark MACHO passes in front of a more distant star, the MACHO's gravity should act like a lens to focus the star's light. Seen from Earth, the star should appear to undergo a characteristic temporary brightening.
Charles Alcock at the Lawrence Livermore National Laboratory in California leads an American-Australian team that has seen at least one probable example of this kind of gravitational lensing. Michelle Spiro and James Rich at the National Center for Scientific Research facility at Saclay near Paris have reported another such sighting. Both teams are using the nearby companion galaxy - the Large Magellanic Cloud - as a source of background stars that may be eclipsed by MACHOs in our own galaxy. Modern electronic and photographic techniques allow them to monitor several million Magellanic Cloud stars at once.
At this time, astronomers are following Dr. Alcock's advice to take these initial discoveries with caution. The researchers need to thoroughly study their data to rule out any errors and to find other examples of lensing before being sure they have spotted the MACHO form of dark matter.
Astronomer Robin Ciardullo of Pennsylvania State University at University Park, who searches for dark matter in distant galaxies, explains that astronomers are like cooks trying to assess a pie they cannot touch. They know the ingredients that make up the crust. But they don't know whether there are apples, blueberries, or some other filling inside. All that astronomers have learned from observing the cosmos through radio, infrared, optical, X-ray, and gamma-ray emissions corresponds to knowledge of the pie crust ingredients. But when it comes to the filling - the dark matter known only by its gravitational effects - nobody has had ``the foggiest notion'' of what it really is, Dr. Ciardullo says.
Thus, he adds, if MACHOs turn out to account for much of the dark matter in our galaxy it will be ``extremely important.'' Also, if MACHOs account for much of the total dark matter in the universe, this will upset current theories of how the cosmos has evolved.