**2.1 Galaxies and Clusters: Dynamical Measures & Mass-to-Light
Ratios**

The contribution of galaxies to the mass density can be determined by
integrating the luminosity function per unit volume for galaxies and
multiplying by an (assumed, constant) mean mass-to-light (*M / L*) ratio.
The dynamical masses of galaxies can be determined from rotation
curves for spiral galaxies, or the measurement of velocity dispersions
and application of the virial theorem both for individual elliptical
galaxies. The latter method can also be applied for groups and
clusters of galaxies (as Zwicky did in the 1930's).

This method has several advantages. First it is conceptually simple
and model-independent. Unlike some of the global techniques discussed
below, this method is independent of both *H*_{0} and
_{}. However, there are a
number of underlying
assumptions. Most important is *the assumption that galaxies
trace all mass*. In addition, there are implicit, underlying
assumptions concerning the similarity of mass-to-light ratios in
different systems (ignoring, for example, potential differences in
initial mass functions, star formation histories, dark remnant
populations, dust content, etc.) The estimates based on this method
tend to yield low values of _{m} of 0.25.