Mass is one of the most basic properties of stellar and galactic systems, so it is natural that astronomers have developed several methods to determine it from observations. Unfortunately, results of different methods often disagree. The most notorious examples are the estimates of masses of clusters of galaxies. A voluminous, if not massive, literature has grown to surround this controversy. On the one hand, the mass can be estimated by independently finding the mass–luminosity ratio of nearby galaxies of each morphological type – spiral, elliptical, or irregular. Counting galaxies of each type in a distant cluster and measuring their luminosities then gives the total mass of the cluster galaxies. On the other hand, the mass of a cluster can be estimated by measuring the galaxy separations and velocities and applying the virial theorem. This dynamical mass usually turns out to be greater than the luminosity mass, often by a factor ~ 10 but occasionally by a factor ~ 100.

Several possible causes may contribute to this discrepancy. The mass–luminosity ratio of nearby galaxies may be intrinsically different from those in more distant groups. There, accretion, tidal interactions, or gaseous ablation, for example, may have altered galaxies even if they were similar to begin with. The mass–luminosity ratio of nearby galaxies may have been underestimated, particularly if they contain extended haloes of dark matter – gas, dust, low luminosity stars, black holes, massive neutrinos or other ʾinos, pebbles, etc.