The galaxy bias is the degree to which the density of galaxies does not reflect the density of dark matter. Given the larger mass of dark matter than stars, galaxies are expected to be in the region of dark matter and are used as a proxy in studies locating dark matter. Surveys of dark matter over time have been done by counting and sizing distant galaxies.
But while presence of galaxies does imply the presence of dark matter, the relative amounts can differ in different regions or times, meaning simple surveys of galaxies give a biased view of dark matter density.
To put it another way, as dark matter is an explanation for the observable dynamics of galaxies and galaxy clusters, 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.