galaxy bias

In discussions of dark matter, the term galaxy bias has been used for the degree to which the mass-density suggested by observed galaxies does not correspond proportionally to the mass density of dark matter in that region of space. One way to discern the amount of dark matter in some region of space is by observing the galaxies in that region. Throughout the observed universe, there would be an observed "mean ratio" between the mass-density implied by the observed galaxies, and that of dark matter. However, in particular locations, this ratio of densities differs from this mean. Characterizing this variation has been an area of interest in cosmology and galaxy formation, to quantify it and work out the factors that relate to it.

Given that the total mass of dark matter far exceeds that of stars, galaxies are expected to be within regions of dark matter and are used as a proxy in studies of the location of dark matter. Surveys of dark matter have been carried out by counting and sizing distant galaxies. However, estimating the a region's dark-matter density by multiplying the mass of the region's galaxies by some single ratio is merely a rough estimate. Dark matter is the explanation for the observable dynamics of galaxy clusters, and that of galaxies themselves, and the study of these dynamics yields the theorized density of dark matter. However, the density of dark matter within a region of space implied by that method is not consistently reflected in the galaxy sizes and populations within that region.