Most of the gravitating matter in the universe is 'dark'. Galaxies are grouped into clusters and super-clusters and it is important to know whether the dark matter is distributed the same way. There are many reasons for suspecting that baryons may have condensed more efficiently into galaxies in regions of above-average density. The overall matter distribution would then be smoother than one would infer on the assumption that galaxies trace mass. The data are incompatible with the hypothesis that the Universe has the critical density unless galaxy formation was biased in this sense.