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4.6.5. Direct Detection of Dark Matter

By definition, the detection of dark matter can only be done indirectly through its gravitational effects. In a broad sense, the very existence of galaxies may signify a detection of dark matter as baryons by themselves seem unlikely candidates for producing galaxy size potentials. The amplitude of the large scale deviations from Hubble flows is quite sensitive to the variation of deltarho / rho over different scale sizes and their existence (still determined at a somewhat low signal-to-noise level) also suggests a large scale dark matter component to the mass distribution. The discovery of gravitational lenses represents another manner in which to detect dark matter as its the total lensing mass which is responsible for the observed degree of distortion of background images. If gravitational lensing by distant clusters of galaxies can be detected (e.g., Squires et al. 1995, Fischer et al. 1996) then it might be possible to search locally for gravitational microlensing and thus constrain the local space density of dark masses than can act as mini-lenses. This idea forms the basis for the MACHO (MAssive Compact Halo Objects) survey which we now describe.