The dark matter problem in disc galaxies

In the generic CDM cosmogony, dark-matter haloes emerge too lumpy and centr ally concentrated to host observed galactic discs. Moreover, discs are pred icted to be smaller than those observed. We argue that the resolution of th ese problems may lie with a combination of the effects of protogalactic dis cs, which would have had a mass comparable to that of the inner dark halo a nd be plausibly non-axisymmetric, and of massive galactic winds, which at e arly times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intraclu ster gas through the G-dwarf problem, point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disc will be the relic of a massive outflow. The inner halo expand ed after absorbing energy and angular momentum from the ejected material. O bserved discs formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budge t and strong radial streaming of the gas had died down.