FIGURE 2.20 The first gravitational microlensing light curve, showing the amplification of the light of a background star by the gravitational field of an intervening object. These intervening objects, of unknown nature, may contribute to the dark matter in the Galaxy. The similarity of the curves in red light and blue light helps confirm that the brightening is caused by gravitational lensing. Courtesy of the MACHO collaboration. Reprinted by permission from Nature 365:621-623, copyright 1993 Macmillan Magazines Ltd.

ongoing efforts in the United States: (1) the Cryogenic Dark Matter Search II, a search for a particle with roughly atomic mass called the neutralino, and (2) the U.S. Axion Experiment, a search for an extremely light dark matter particle called the axion. The existence of the neutralino is a prediction of superstring theory, a bold and promising attempt to unify gravity with the other forces of nature. The discovery that neutralinos or axions are the dark matter that binds our own galaxy would shed light not only on the astrophysical dark matter problem, but also on the unification of the fundamental forces and particles of nature.

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