ground-based optical and radio telescopes may yield better maps of the location of dark matter.

Dark matter will also be mapped in the coming decade by x-ray emission from hot gas. Very hot gas has been detected inside large clusters of galaxies extending 5 million to 10 million light-years out from the center of many clusters. So hot that it should boil away, the gas is evidently held by the gravity of invisible matter. From the precise distribution of the gas, astronomers can work back to infer the gravity confining it and the distribution of dark matter producing that gravity. In the coming decade, the German X-ray Roentgen Satellite (ROSAT) already in orbit, the Japanese x-ray satellite ASTRO-D, and especially NASA's AXAF telescope will make better maps of the distribution and temperature of the hot gas in galaxy clusters.

The identification of dark matter may be a process of elimination. For example, the dark matter could be large planets, with masses between a thousandth and a tenth the mass of our sun. Such objects should have enough heat generated by their slow contraction to emit a low intensity of infrared radiation. A highly sensitive infrared telescope like SIRTF may be able to detect them. In particular, SIRTF will scrutinize the far reaches of our own galaxy, where dark matter may be lurking, and search for a faint excess glow.

There are other ways to probe dark matter. One of the most recent and potentially very important new techniques makes use of the “ gravitational lens” phenomenon. When he published his new theory of gravity, Einstein pointed out that light, like matter, should be affected by gravity. Thus as light from a distant astronomical object, such as a quasar, travels toward the earth, that light should be deflected by any mass lying between here and there. The intervening mass can act as a lens, distorting and splitting the image of the quasar. Even if the intervening mass is totally invisible, its gravitational effects are not. By carefully analyzing the distortions of quasar images, astronomers can reconstruct many of the properties of the intervening gravitational lens, including its distribution in space and total mass. Gravitational lenses were first discovered only a decade ago; about a dozen have been found since that time. In the coming decade, the gravitational-lens phenomenon will be used as a powerful tool to uncover the nature of dark matter.

Alternatively, dark matter could consist of individual, freely roaming sub-atomic particles, rather than aggregates of particles such as planets. The possibilities have stirred the imaginations of particle theorists. Dozens of particles have been proposed, some on the basis of new theories of subatomic physics. None, however, have as yet been seen in the laboratory. If dark matter does indeed consist of these exotic particles, then it may be identified in the laboratory rather than at the telescope. Within the last few years, the first laboratory detectors have been built to search for some of these hypothesized particles. The experiments are extremely difficult, owing to the elusiveness of



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