The galaxy bias is the degree to which the density of galaxies does not reflect the density of dark matter. 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 over time have been done 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 a single ratio is merely a rough estimate, which varies in accuracy. Dark matter is the explanation for the observable dynamics of galaxies and galaxy clusters and sufficient study of a galaxy yields the theorized amount of dark matter, but this amount is not consistently reflected by the size of the galaxy or density of galaxies in a region.