Background image: The Bullet Cluster, flanked by dark matter. Learn more >
In the 1930s it was observed that the orbital velocities of stars in our galaxy, and the speed of galaxies within clusters was out of line with their apparent mass. Oort and Zwicky postulated that the 'missing mass' was composed of particles that only interact gravitationally with visible matter. Strong supporting evidence for this idea came in the 1960s when it was observed that the Bullet cluster exhibited gravitational lensing effects consistent with the existence of dark matter.
In the standard lambda-CDM (cold, dark matter) model dark matter makes up around 27% of the current energy content of the universe, with standard-model matter making up just 5%, the rest dark energy. These models are in excellent accord with anisotropies revealed in the cosmic microwave background by COBE. Whether they will remain in agreement with observations made by PLANCK is an open question.
An alternative explanation that enjoyed currency among some cosmologists over the last two decades is the hypothesis that the laws of gravity need to be modified in the very weak field regime. In particular, Mordehai Milgrom in 1983 proposed a modification that fit the known data on galaxy and cluster rotation rates extremely well. This approach, called MOND (for Modified Newtonian Dynamics), is likely to be conclusively disproved if evidence for dark matter continues to accumulate.