As mentioned above, the first evidence for dark matter came from a study of galactic clusters.  In 2006, an investigation of the Bullet Cluster, which is composed of two colliding clusters of galaxies, provided important evidence for the existence of dark matter. 

This cluster, about 3.4 billion light-years away, is unusual in that most of its matter consists of hot gases (see image). 

This image of the Bullet cluster shows a separation of dark matter from the luminous matter, with the dark matter shown in blue and the luminous matter shown in red. Image credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

1. Measurement of the emitted electromagnetic radiation indicated that most of the ordinary matter in the system was in the form of hot gases, which emit x-rays.
2. Measurement of the distribution of x-ray emission produced the distribution of hot gases.
3. A study of gravitational lensing (see previous section) determined the distribution of all the mass in the pair of colliding clusters.

During the collision, the colliding gases--ordinary matter-- interact through electrostatic forces between atoms and molecules, which slows their passage through each other. The dark matter in each galaxy, on the other hand, interacts only through the much weaker gravitational force, so its passage was not slowed by electrostatic interactions and over time it pulled ahead of the gas. The result of this investigation is shown in the image:

• blue regions show the distribution of the total mass, determined from gravitational lensing
• red regions show the hot gases, determined from x-ray emission

Since the red regions are separate from the blue regions, the vast preponderance of matter in the galaxy must be located in blue regions, since all matter in the cluster reacts gravitationally and thus contributes to gravitational lensing.  This observation is taken to be the first direct evidence for dark matter, and this conclusion is now supported by a second example of dark matter-ordinary matter separation in colliding clusters (see Links).