Changes

[[File:lighterstill.jpg]][[File:Dark-matter.jpg|right|frame]]

[[File:lighterstill.jpg]][[File:Dark-matter.jpg|right|frame]]

In [[astronomy]] and [[cosmology]], '''dark matter''' is a hypothetical [[form]] of matter that is undetectable by its emitted [http://en.wikipedia.org/wiki/Electromagnetic_radiation electromagnetic radiation], but whose [[presence]] can be [[infer]]red from [[gravitational]] [[effects]] on visible [[matter]].[1] According to [[present]] [[observations]] of [[structures]] larger than galaxies, as well as [http://en.wikipedia.org/wiki/Big_bang Big Bang cosmology], dark matter and dark [[energy]] could account for the vast majority of the (missing) [[mass]] in the observable universe.

In [[astronomy]] and [[cosmology]], '''dark matter''' is a hypothetical [[form]] of matter that is undetectable by its emitted [https://en.wikipedia.org/wiki/Electromagnetic_radiation electromagnetic radiation], but whose [[presence]] can be [[infer]]red from [[gravitational]] [[effects]] on visible [[matter]].[1] According to [[present]] [[observations]] of [[structures]] larger than galaxies, as well as [https://en.wikipedia.org/wiki/Big_bang Big Bang cosmology], dark matter and dark [[energy]] could account for the vast majority of the (missing) [[mass]] in the observable universe.

Dark matter was postulated by [http://en.wikipedia.org/wiki/Fritz_Zwicky Fritz Zwicky] in 1934, to partially account for [[evidence]] of "missing mass" in the universe, including the [http://en.wikipedia.org/wiki/Galaxy_rotation_problem rotational speeds of galaxies], orbital velocities of galaxies in clusters, [http://en.wikipedia.org/wiki/Gravitational_lensing gravitational lensing] of background objects by [[galaxy]] clusters such as the [http://en.wikipedia.org/wiki/Bullet_Cluster Bullet Cluster], and the temperature [[distribution]] of hot gas in galaxies and clusters of galaxies. Fritz Zwicky is the "Father of Dark Matter," coining the term itself, as well as gravitational lensing and the sky survey [[technique]].

Dark matter was postulated by [https://en.wikipedia.org/wiki/Fritz_Zwicky Fritz Zwicky] in 1934, to partially account for [[evidence]] of "missing mass" in the universe, including the [https://en.wikipedia.org/wiki/Galaxy_rotation_problem rotational speeds of galaxies], orbital velocities of galaxies in clusters, [https://en.wikipedia.org/wiki/Gravitational_lensing gravitational lensing] of background objects by [[galaxy]] clusters such as the [https://en.wikipedia.org/wiki/Bullet_Cluster Bullet Cluster], and the temperature [[distribution]] of hot gas in galaxies and clusters of galaxies. Fritz Zwicky is the "Father of Dark Matter," coining the term itself, as well as gravitational lensing and the sky survey [[technique]].

<center>For lessons on the [[topic]] of '''''Dark Matter''''', follow [https://nordan.daynal.org/wiki/index.php?title=Category:Dark_Matter '''''this link'''''].</center>

<center>For lessons on the [[topic]] of '''''Dark Matter''''', follow [https://nordan.daynal.org/wiki/index.php?title=Category:Dark_Matter '''''this link'''''].</center>

Dark matter is believed to play a central role in [[structure]] formation and galaxy [[evolution]], and has [[Measure|measurable]] [[effects]] on the [http://en.wikipedia.org/wiki/Anisotropy anisotropy] of the [http://en.wikipedia.org/wiki/Cosmic_microwave_background 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 remainder is frequently called the "dark matter component," even though there is a small amount of [http://en.wikipedia.org/wiki/Baryonic_dark_matter baryonic dark matter]. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly [[transparent]].

Dark matter is believed to play a central role in [[structure]] formation and galaxy [[evolution]], and has [[Measure|measurable]] [[effects]] on the [https://en.wikipedia.org/wiki/Anisotropy anisotropy] of the [https://en.wikipedia.org/wiki/Cosmic_microwave_background 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 remainder is frequently called the "dark matter component," even though there is a small amount of [https://en.wikipedia.org/wiki/Baryonic_dark_matter baryonic dark matter]. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly [[transparent]].

The vast majority of the dark matter in the universe is believed to be [http://en.wikipedia.org/wiki/Baryon nonbaryonic], which means that it contains no [[atoms]] and that it does not interact with ordinary matter via [http://en.wikipedia.org/wiki/Electromagnetic_force electromagnetic forces]. The nonbaryonic dark matter includes [http://en.wikipedia.org/wiki/Neutrino neutrinos], and possibly hypothetical [[entities]] such as [http://en.wikipedia.org/wiki/Axion axions], or [http://en.wikipedia.org/wiki/Supersymmetry supersymmetric particles]. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe ("[http://en.wikipedia.org/wiki/Big_bang_nucleosynthesis 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 undergo annihilation [[interaction]]s with themselves resulting in [[observable]] by-products such as photons and neutrinos ("indirect detection").[3]

The vast majority of the dark matter in the universe is believed to be [https://en.wikipedia.org/wiki/Baryon nonbaryonic], which means that it contains no [[atoms]] and that it does not interact with ordinary matter via [https://en.wikipedia.org/wiki/Electromagnetic_force electromagnetic forces]. The nonbaryonic dark matter includes [https://en.wikipedia.org/wiki/Neutrino neutrinos], and possibly hypothetical [[entities]] such as [https://en.wikipedia.org/wiki/Axion axions], or [https://en.wikipedia.org/wiki/Supersymmetry supersymmetric particles]. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe ("[https://en.wikipedia.org/wiki/Big_bang_nucleosynthesis 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 undergo annihilation [[interaction]]s with themselves resulting in [[observable]] by-products such as photons and neutrinos ("indirect detection").[3]

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 [http://en.wikipedia.org/wiki/Hot_dark_matter Hot Dark Matter] (HDM), [http://en.wikipedia.org/wiki/Warm_dark_matter Warm Dark Matter] (WDM), and [http://en.wikipedia.org/wiki/Cold_dark_matter Cold 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 a [http://en.wikipedia.org/wiki/Neutralino neutralino]. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.[4]

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 [https://en.wikipedia.org/wiki/Hot_dark_matter Hot Dark Matter] (HDM), [https://en.wikipedia.org/wiki/Warm_dark_matter Warm Dark Matter] (WDM), and [https://en.wikipedia.org/wiki/Cold_dark_matter Cold 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 a [https://en.wikipedia.org/wiki/Neutralino neutralino]. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.[4]

As important as dark matter is believed to be in the [[universe]], direct [[evidence]] of its [[existence]] and a concrete [[understanding]] of its [[nature]] have remained elusive. Though the [[theory]] of dark matter remains the most widely accepted theory to explain the [[Anomaly|anomalies]] in observed galactic rotation, some alternative theories such as [http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics modified Newtonian dynamics] and [http://en.wikipedia.org/wiki/Tensor-vector-scalar_gravity tensor-vector-scalar gravity] have been proposed. None of these alternatives, however, has garnered equally widespread support in the [[scientific]] [[community]].[http://en.wikipedia.org/wiki/Dark_gravity]

As important as dark matter is believed to be in the [[universe]], direct [[evidence]] of its [[existence]] and a concrete [[understanding]] of its [[nature]] have remained elusive. Though the [[theory]] of dark matter remains the most widely accepted theory to explain the [[Anomaly|anomalies]] in observed galactic rotation, some alternative theories such as [https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics modified Newtonian dynamics] and [https://en.wikipedia.org/wiki/Tensor-vector-scalar_gravity tensor-vector-scalar gravity] have been proposed. None of these alternatives, however, has garnered equally widespread support in the [[scientific]] [[community]].[https://en.wikipedia.org/wiki/Dark_gravity]

[[Category: Physical Cosmology]]

[[Category: Physical Cosmology]]