Astronomy Colloquium

The cold dark matter paradigm is being challenged by its apparent inability to explain the low density measured at the centre of cosmological systems, ranging from faint dwarf galaxies to massive clusters containing hundreds of galaxies the size of the Milky Way.  But before drawing conclusions one should carefully include the effect of gas and stars, which were historically seen as merely a passive component during the assembly of galaxies. We now understand that these can in fact significantly alter the dark matter component, through a coupling based on rapid gravitational potential fluctuations. I will review recent observational and theoretical developments and take a critical look at just how little we understand about the dynamics of dark matter in even the simplest scenarios.