Un-controversial thread (no dissenters please)

This should explain it:

In astrophysics, dark matter is matter that does not emit or reflect enough electromagnetic radiation (such as light, X-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. Among the observed phenomena consistent with the existence of dark matter are the rotational speeds of galaxies and 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. Dark matter also plays a central role in structure formation and Big Bang nucleosynthesis, and 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 is directly observable, indicating that the remainder is dark.

The composition of dark matter is unknown, but may include new elementary particles such as WIMPs and axions, ordinary and heavy neutrinos, dwarf stars and planets collectively called MACHOs, and clouds of nonluminous gas. Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called nonbaryonic dark matter.

The dark matter component has vastly 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 metre of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some hard-to-detect baryonic matter (see baryonic dark matter) makes a contribution to dark matter, but constitutes only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that dark matter and dark energy serve mainly as expressions of our ignorance, much as the marking of early maps with terra incognita.


Copied from wikipedia.