Dark Matter

In astrophysics and physical cosmology, dark matter is referred to the hypothetical matter that does not emit enough electromagnetic radiation to be detected with current technology, but whose existence can be inferred from gravitational effects on visible matter cause, such as stars or galaxies, and in the anisotropies of the cosmic microwave background in the universe. Do not confuse dark matter with dark energy.
According to current observations (2010) of structures larger than a galaxy, as well as Big Bang cosmology, dark matter is of the order of 21% of the mass of the observable universe and dark energy 70%.
Dark matter was proposed by Fritz Zwicky in 1933 to evidence of "invisible mass" affecting the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe: these observations include that speed of rotation of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster (1E 0657 - 56) and the temperature distribution of hot gas in galaxies and clusters of galaxies.
Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the microwave background radiation. All these evidence suggest that galaxies, clusters of galaxies and the entire universe contains far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component."
The composition of dark matter is unknown, but may include ordinary and heavy neutrinos, recently postulated elementary particles such as WIMPs and axions, astronomical bodies such as dwarf stars, planets (collectively called MACHO) and non-luminous gas clouds. Current evidence favors models in which the primary component of dark matter is new elementary particles collectively called non-baryonic dark matter.
The dark matter component has much more mass than the component "visible" of the Universe. At present, ordinary bariones density and radiation in the universe are estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 5% of the total energy density in the Universe (inferred from gravitational effects) can be seen directly. It is estimated that around 23% is composed of dark matter. The remaining 72% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some difficult to detect baryonic matter makes a contribution to the dark matter, although some authors argue that is only a small portion. Even so, it should be noted that 5% of estimated baryonic matter (half of it has not been detected) can be considered baryonic dark matter: All the stars, galaxies and gas observable form less than half of the baryons (which is supposed to be) and it is believed that all this stuff can be distributed at low density gaseous filaments forming a network throughout the universe and whose nodes are the various clusters of galaxies. In May 2008, the XMM-Newton telescope of the European Space Agency has found evidence of the existence of such a network of filaments.
The determination of the nature of this invisible mass is one of the most important issues of modern cosmology and particle physics. It has been shown that the names "dark matter" and "dark energy" serve mainly as expressions of our ignorance, almost like the first maps labeled "Terra incognita".