  To the astrophysicist faced with the puzzle of dark matter, this one appears
under two different aspects: on the one hand in cosmology, i.e. at very large
scales, where it seems to be made of a bath of particles; on the other hand at
the scale of galaxies, where it is described by a set of specific phenomena,
looking incompatible with a description in terms of particles, and suggesting
that we are seeing a modification of the law of gravity. Reconciling these two
distinct aspects of dark matter in a single theoretical formalism is an
important challenge which could lead to some new physics in action at
astronomical scales.
