Everything presented so far in this review has dealt with the luminosity function of galaxies, which is not the same as the mass function (by mass function here I am referring to total mass, where the dark matter content of individual galaxies is included). The two, however, probably are fairly similar, unless there exist a significant dispersion in the mass-to-light vs. light distribution function for galaxies in clusters. Clusters of galaxies possess a great deal of dark matter so that it is not impossible that they contain many galaxies that are almost completely dark. The mass function would be a useful probe of the fluctuation spectrum, since it bypasses all the complex physics of the baryonic component outlined in Section 1 (in the cores of rich clusters, however, there remains the caveat that the dark-matter properties of galaxies might be significantly affected by cluster-related processes e.g. tidal stripping).
The mass function is not straightforward to measure. The best hope is to determine it through gravitational lensing measurements, for example (i) galaxy-galaxy strong lensing, or (ii) the Natarajan-Kneib  method, which attributes granularity in the weak lensing shear map to lensing by individual galaxy halos. In the upcoming age of 8 and 10 m telescopes, it is likely that these kinds of measurements will provide important constraints.