Page "Galaxy groups and clusters" Paragraph 5

Clusters are larger than groups, although there is no sharp dividing line between the two.

When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction.

However, their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity.

X-ray studies have revealed the presence of large amounts of intergalactic gas known as the intracluster medium.

This gas is very hot, between 10 < sup > 7 </ sup > K and 10 < sup > 8 </ sup > K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission.

The total mass of the gas is greater than that of the galaxies by roughly a factor of two.

However this is still not enough mass to keep the galaxies in the cluster.

Since this gas is in approximate hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined.

It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas.

The missing component is known as dark matter and its nature is unknown.

In a typical cluster perhaps only 5 % of the total mass is in the form of galaxies, maybe 10 % in the form of hot X-ray emitting gas and the remainder is dark matter.

Brownstein and Moffat use a theory of modified gravity to explain X-ray cluster masses without dark matter.

Observations of the Bullet Cluster are the strongest evidence for the existence of dark matter ; however, Brownstein and Moffat have shown that their modified gravity theory can also account for the properties of the cluster.