Gravitational microlensing and dark matter in the galactic halo

We present the basics of microlensing and give an overview of the results obtained so far. We also describe a scenario in which dark clusters of MACHOs (Massive Astrophysical Compact Halo Objects) and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10-20 kpc. Moreover, we discuss various experimental tests of this picture in particular a $\gamma$-ray emission from the clouds due to the scattering of high-energy cosmic-ray protons. Our estimate for the $\gamma$-ray flux turns out to be in remarkably good agreement with the recent discovery by Dixon et al. of a possible $\gamma$-ray emission from the halo using EGRET data.