Transcript: Another way to look at the mass density of the universe is in terms of the cosmic mass to light ratio. Mass to light ratio is defined as the ratio of the mass, in solar units, to the luminosity, in solar units, so for the Sun by definition M over L is one. In general, low mass stars have mass to light ratios greater than one, and high mass stars have ratios less than one. We know that the stellar populations typical of normal galaxies give overall mass to light ratios in the range of three to ten. This means that anywhere in the universe on larger scales where we infer mass to light ratios much above ten we must be looking at dark matter. The halos in galaxies through dynamical measurements have mass to light ratios in the range ten to fifty, groups of galaxies in the range thirty to two hundred, clusters of galaxies in the range one hundred to four hundred, and the Local Supercluster, through a very uncertain measurement, has a mass to light ratio in the range three hundred to five hundred. On all these large scales most of the mass is coming from dark matter. However, the mass to light ratio in the standard cosmology corresponding to critical mass density is one thousand five hundred. So even on the largest scales there’s not sufficient dark matter to account for a critical density; it’s a factor of three short.