Microlensing and Halo Cold Dark Matter

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Abstract
We discuss the implications of the more than 50 microlensing events seen for the composition of the dark halo of our galaxy. Though firm conclusions are not yet possible due to small-number statistics and modeling uncertainties, in most viable models of the galaxy the fraction of massive compact objects in the halo (MACHOs) is between 5% and 30%, consistent with expectations for a universe whose primary component is cold dark matter. An all-MACHO halo cannot yet be excluded; we discuss future measurements that could exclude this possibility.