  The size and time of formation of the first gravitationally bound objects in
the Universe is set by the microphysical properties of the dark matter. It is
argued that observations seem to favour cold and thermal candidates for the
main contribution to the dark matter. For that type of dark matter, the size
and time of formation of the first halos is determined by the elastic cross
sections and mass of the CDM particles. Consequently, the astrophysics of CDM
might allow us to measure some of the fundamental parameters of CDM particles.
Essential for observations is the survival rate and spatial distribution of the
very first objetcs, which are currently under debate.
