Galaxy mass-light ratio

A typical mass light ratio of 5 solar units for stars and detected gas in galaxies thus contributes only <2 Um = 0.003A—1 to the smoothed-out mass density, corresponding to 7710 < 1. 

Abundance ratios and the Iron Mass to Light Ratio (IMLR) are good tests for the evolution of galaxies in clusters since they do not depend on the total cluster gas mass. The IMLR is defined as (Renzini et al. 1993) ... 

In order to infer the mass density of the universe, one has to make the assumption that the ratio of dark matter to light is the same everywhere in the universe. This is far from being obvious, as clusters represents only 10-5 of the total volume of the universe. Evidences for the presence of a such large quantity of dark matter are probably reasonable but far from being as robust as in clusters. A dramatic possibility would be that dark matter is present in large quantity only in clusters... (the amount of dark matter directly seen in galaxies from rotation curves is much smaller than in clusters). However, there are a... 

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