The missing mass problem in galaxies
Prof. Benoit Famaey (CNRS, Strasbourg)

A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. On cosmological scales, the Universe is well described by a model including a so-called "cold dark matter" component and a cosmological constant. There however remain some heavy clouds on our global understanding of mass discrepancies, 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 could 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. Current alternatives to the standard cosmological model however bring with them many unsolved (and perhaps unsolvable) questions and challenges.