| Description: | The cold dark matter picture can be tested by carefully measuring the kinematics of internal motions in galaxies. Especially at the scale of faint dwarf galaxies, HI rotation curves challenge the paradigm: the density inferred is smaller than that expected. A similar problem may be arising at the scale of massive clusters containing hundreds of galaxies the size of the Milky Way. Does this mean we are seeing evidence for exotic dark matter? 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. Nonetheless there is hope that future measurements can determine the nature of the dark matter particle.
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