In cosmology, dark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation (light) to be detected directly, but whose presence may be inferred from gravitational effects on visible matter such as stars and galaxies. Dark matter explains several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter also resolves a number of seeming inconsistencies in the Big Bang theory, and is crucial for structure formation.
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 meter 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. Its urgency is underlined by David B. Cline in a 2003 article in Scientific American, in which he writes: "The terms . . . 'dark matter' and 'dark energy,' serve mainly as expressions of our ignorance", much as the marking of early maps with 'Terra Incognita'.
- Hot News for Cold Dark Matter
- Most of Our Universe is Missing
- David B. Cline, The Search for Dark Matter, in Scientific American, March 2003
- Hot Dark Matter - by Martin White
- Dark Matter Portal - by Frédéric Mayet
- "Most of our Universe is Missing" - Horizon video
- "Particle Dark Matter: Evidence, Candidates and Constraints" - by G. Bertone, D. Hooper and J. Silk
- "Experimental Searches for Dark Matter" - by Timothy J. Sumner
- "Dark matter comes out of the cold" - BBC News 5 February 2005
- "Astronomers find Starless Galaxy" - BBC News 23 February 2005
- "In the Beginnings" - The Economist 13 January 2005
- Elliptical galaxies have dark matter halo as well
- Recent papers on dark matter on arXiv.org