  Two exotic elements have been introduced into the standard cosmological
model: non-baryonic dark matter and dark energy. The success in converting a
hypothesis into a solid theory depends strongly on whether we are able to solve
the problems in explaining observations with these dark elements and whether
the solutions of these problems are unique within the standard paradigm without
recourse to alternative scenarios. We have not achieved that success yet
because of numerous inconsistencies, mainly on galactic scales, the
non-detection so far of candidate particles for dark matter, and the existence
of many alternative hypotheses that might substitute the standard picture to
explain the cosmological observations. A review of some ideas and facts is
given here.
