  One of the most important problems in astrophysics concerns the nature of the
dark matter in galactic halos, whose presence is implied mainly by the observed
flat rotation curves in spiral galaxies. Due to the Pauli exclusion principle
it can be shown that neutrinos cannot be a major constituent of the halo dark
matter. As far as cold dark matter is concerned there might be a discrepancy
between the results of the N-body simulations and the measured rotation curves
for dwarf galaxies. A fact this, if confirmed, which would exclude cold dark
matter as a viable candidate for the halo dark matter.
  In the framework of a baryonic scenario the most plausible candidates are
brown or white dwarfs and cold molecular clouds (mainly of $H_2$). The former
can be detected with the ongoing microlensing experiments. In fact, the French
collaboration EROS and the American-Australian collaboration MACHO have
reported the observation of altogether $\sim$ 10 microlensing events by
monitoring during several years the brightness of millions of stars in the
Large Magellanic Cloud. In particular, the MACHO team announced the discovery
of 8 microlensing candidates by analysing their first 2 years of observations.
This would imply that the halo dark matter fraction in form of MACHOs (Massive
Astrophysical Compact Halo Objects) is of the order of 45-50% assuming a
standard spherical halo model. The most accurate way to get information on the
mass of the MACHOs is to use the method of mass moments, which leads to an
average mass of 0.27$M_{\odot}$.
