  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.
