  The Universe on large scales is well described by the Lambda-CDM cosmological
model. There however remain some heavy clouds on our global understanding,
especially on galaxy scales, which we review here. While some of these clouds
might perhaps disappear through small compensatory adjustments of the model,
such as changing the mass of the dark matter particles or accounting better for
baryonic physics, others should rather be taken as strong indications that the
physics of the dark sector is, at the very least, much richer and complex than
currently assumed, and that our understanding of gravity and dynamics might
also be at play. For instance, the empirically well-tested MOND phenomenology
in galaxies, whatever its final explanation, should be understood in any model
of galaxy formation and dynamics. Current alternatives to Lambda-CDM however
bring with them many unsolved questions and challenges.
