Density of Luminous Matter

The density of luminous matter in the universe seems to be about 5 x 10^-28 kg/m³ or 0.28 GeV/m³. This is about one tenth of the critical density of 47 x 10^-28kg/m³ or 2.6 GeV/m³ which is obtained by using a Hubble parameter of 50 km/s/Mpc. Since the observed expansion rate suggests that the universe is close to critical density, there must be a large amount of unseen matter, generally lumped under the term "dark matter".

Dark Matter in the Universe

The density of luminous matter seen in the sky seems to be only about one-tenth the critical density required to "close" the universe. Yet the evidence on the expansion of the universe suggests that we are close to critical density, so there is considerable discussion of the "dark matter" which must be present.

One indication of the amount of dark matter in our galaxy comes from the measurement of the relative speeds of stars. When that data is used to model the total mass of our galaxy, it turns out to be about an order of magnitude larger than the mass of the luminous stars in the galaxy.

On a larger scale, the measurement of the speeds of galaxies within clusters gives a value for the total mass of the cluster. Again the inferred total mass seems to be about an order of magnitude larger than the visible mass of the galaxies.

The current assessment is that about 90% of the mass of the universe is in the dark matter. The nature of this dark matter is not known.