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Dark matter

In astronomyandcosmology,dark matter is hypotheticalmatterthat is undetectable by its emittedradiation, but whose presence can be inferred fromgravitationaleffects on visible matter. Dark matter is postulated to explain the flatrotation curvesofspiral galaxiesand other evidence of "missing mass" in the universe. According to present observations of structures larger thangalaxies, as well asBig Bang cosmology, dark matter anddark energyaccount for the vast majority of the mass in theobservable universe. The observed phenomena which imply the presence of dark matter include therotational speeds of galaxies, orbital velocities ofgalaxiesinclusters,gravitational lensingof background objects by galaxy clusters such as theBullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role instructure formationandgalaxy evolution, and has measurable effects on theanisotropyof thecosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter thanthat which interacts with electromagnetic radiation: the remainder is frequently called the "dark matter component," even though there is a small amount ofbaryonic dark matter.

The dark matter component has much more mass than the "visible" component of the universe. At present, the density of ordinarybaryonsandradiationin 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 ofdark energy, an even stranger component, distributed diffusely in space. Some hard-to-detectbaryonic matteris believed to make a contribution to dark matter but would constitute only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology andparticle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita."

The vast majority of the dark matter in the universe is believed to be nonbaryonic, which means that it contains no atomsand that it does not interact with ordinarymatterviaelectromagnetic forces. The nonbaryonic dark matter includesneutrinos, which werediscoveredto have mass in recent years, and may also include hypothetical entities such asaxions, orsupersymmetricparticles. Unlikebaryonic dark matter, nonbaryonic dark matter does not contribute to the formation of theelementsin the early universe ("big bang nucleosynthesis") and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergoannihilationinteractions with themselves resulting in observable by-products such asphotonsand neutrinos ("indirect detection").

Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly). There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter(HDM),Warm Dark Matter(WDM), andCold Dark Matter(CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be aneutralino. Hot dark matter might consist of (massive) neutrinos. Cold dark matter leads to a "bottom-up" formation of structure in the universe while hot dark matter results in a "top-down" formation scenario.

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