Dark matter is hypothetical matter which can not be detected by its emitted radiation. Its presence can only be inferred from gravitational effects on visible matter. The flat rotation curves of spiral galaxies and other evidence of "missing mass" in the universe is due to dark matter.
Dark matter accounts for the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies, playing a central role in galaxy evolution. It also has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation.
The dark matter component has much more mass than the visible component of the universe. At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density in the universe can be seen directly. About 22% is thought to be composed of dark matter.