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'''Dark matter''' is a type of matter thought to be responsible for much of the [[mass]] in the [[universe]].

'''Dark matter''' is a type of matter thought to be responsible for much of the [[mass]] in the [[universe]].

The idea arose when astronomers found that the mass of large astronomical objects, figured out  from their [[gravitational]] effects, was much greater than the mass figured out from the "luminous matter" they contain: [[stars]], [[gas]], and [[dust]].

The idea arose when astronomers found that the mass of large [[Astronomy|astronomical]] objects, figured out  from their [[gravitational]] effects, was much greater than the mass figured out from the "luminous matter" they contain: [[stars]], [[gas]], and [[dust]].

Dark matter was first proposed by [[Jan Oort]] in 1932 as a reason for the spinning speeds of stars in the [[Milky Way]]. [[Fritz Zwicky]] in 1933 used dark matter to explain "missing mass" in the spinning speeds of [[galaxy|galaxies]] in [[galaxy cluster|clusters]]. Later, many other observations have suggested that there is dark matter in the universe. The spinning speeds of galaxies,<ref>[http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm First observational evidence of dark matter]. Darkmatterphysics.com. Retrieved on 6 August 2013.</ref> [[gravitational lensing]] of background objects, the temperature distribution of hot gas in galaxies and clusters of galaxies: these are some of the examples that make scientists believe in dark matter.

Dark matter was first proposed by [[Jan Oort]] in 1932 as a reason for the spinning speeds of stars in the [[Milky Way]]. [[Fritz Zwicky]] in 1933 used dark matter to explain "missing mass" in the spinning speeds of [[galaxy|galaxies]] in [[galaxy cluster|clusters]]. Later, many other observations have suggested that there is dark matter in the universe. The spinning speeds of galaxies,<ref>[http://www.darkmatterphysics.com/Galactic-rotation-curves-of-spiral-galaxies.htm First observational evidence of dark matter]. Darkmatterphysics.com. Retrieved on 6 August 2013.</ref> [[gravitational lensing]] of background objects, the temperature distribution of hot gas in galaxies and clusters of galaxies: these are some of the examples that make scientists believe in dark matter.

According to the [[Planck (spacecraft)#2013 data release|Planck mission team]], and based on the [[standard model of cosmology]], the total [[Mass–energy equivalence|mass–energy]] of the [[observable universe|known universe]] contains 4.9% [[Baryon#Baryonic matter|ordinary matter]], 26.8% dark matter and 68.3% [[dark energy]].<ref name="planck_overview">{{cite journal |first1=P.A.R. |last1=Ade |first2=N. |last2=Aghanim |first3=C. |last3=Armitage-Caplan |last4=''et al''. (Planck Collaboration) |title=Planck 2013 results. I. Overview of products and scientific results&nbsp;– Table 9 |journal=Astronomy and Astrophysics |volume=1303 |pages=5062 |url=http://www.sciops.esa.int/index.php?project=PLANCK&page=Planck_Published_Papers |date=2013 |arxiv=1303.5062 |bibcode=2013arXiv1303.5062P |displayauthors=30 |access-date=2014-07-24 |archive-date=2016-08-13 |archive-url=https://web.archive.org/web/20160813220816/http://www.sciops.esa.int/index.php?project=PLANCK |url-status=dead }}</ref><ref name="wmap7parameters">{{cite web|title = First Planck results: the Universe is still weird and interesting|url =https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/|author=Francis, Matthew |date= 2013|work=Arstechnica}}</ref> So, dark matter is estimated to make up 84.5% <!--26.8/(4.9 + 26.8)--> of the total matter in the universe, while dark energy plus dark matter make up 95.1% of the total "stuff" in the universe.<ref name=planckcam>{{cite web |url=http://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |title=Planck captures portrait of the young Universe, revealing earliest light |date= 2013 |publisher=University of Cambridge  |accessdate=22 March 2013}}</ref><ref>Ferris, Timothy. 2015. Dark Matter. ''National Geographic''. [http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text] {{Webarchive|url=https://web.archive.org/web/20141225013843/http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text|date=2014-12-25}}</ref>

According to the [[Planck (spacecraft)#2013 data release|Planck mission team]], and based on the [[standard model of cosmology]], the total [[Mass–energy equivalence|mass–energy]] of the [[observable universe|known universe]] contains 4.9% [[Baryon#Baryonic matter|ordinary matter]], 26.8% dark matter and 68.3% [[dark energy]].<ref name="planck_overview">{{cite journal |first1=P.A.R. |last1=Ade |first2=N. |last2=Aghanim |first3=C. |last3=Armitage-Caplan |last4=''et al''. (Planck Collaboration) |title=Planck 2013 results. I. Overview of products and scientific results&nbsp;– Table 9 |journal=Astronomy and Astrophysics |volume=1303 |pages=5062 |url=http://www.sciops.esa.int/index.php?project=PLANCK&page=Planck_Published_Papers |date=2013 |arxiv=1303.5062 |bibcode=2013arXiv1303.5062P |displayauthors=30 |access-date=2014-07-24 |archive-date=2016-08-13 |archive-url=https://web.archive.org/web/20160813220816/http://www.sciops.esa.int/index.php?project=PLANCK |url-status=dead }}</ref><ref name="wmap7parameters">{{cite web|title = First Planck results: the Universe is still weird and interesting|url =https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/|author=Francis, Matthew |date= 2013|work=Arstechnica}}</ref> So, dark matter is estimated to make up 84.5% <!--26.8/(4.9 + 26.8)--> of the total matter in the universe, while dark energy plus dark matter make up 95.1% of the total "stuff" in the universe.<ref name=planckcam>{{cite web |url=http://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |title=Planck captures portrait of the young Universe, revealing earliest light |date= 2013 |publisher=University of Cambridge  |accessdate=22 March 2013}}</ref><ref>Ferris, Timothy. 2015. Dark Matter. ''National Geographic''. [http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text] {{Webarchive|url=https://web.archive.org/web/20141225013843/http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/ferris-text|date=2014-12-25}}</ref>

   

   

Because dark matter does not seem to give off or reflect [[light]], [[x-ray]]s, or any other [[electromagnetic radiation|radiation]], the instruments that are used to find normal matter (like hot gas, stars, planets, and us) can't find dark matter. It seems that dark matter is not made of the same thing as the matter we see every day on Earth. The only way we can tell if dark matter is there, is by how it affects things we can "see" by gravity.

Because dark matter does not seem to give off or reflect [[light]], [[x-ray]]s, or any other [[electromagnetic radiation|radiation]], the instruments that are used to find normal matter (like hot gas, stars, planets, and us) can't find dark matter. It seems that dark matter is not made of the same thing as the matter we see every day on [[Earth]]. The only way we can tell if dark matter is there, is by how it affects things we can "see" by gravity.

In 2006, a group of scientists claimed that they had found a way to find dark matter.<ref name="stanford">{{cite journal|title=Dark matter observed|url=http://home.slac.stanford.edu/pressreleases/2006/20060821.htm}}</ref> Since dark matter is supposedly very different from normal matter, it is expected to act differently. The scientists observed two far-away [[galaxy cluster]]s that had crashed into each other at high speed: normal matter would have been scattered nearby after the collision, while dark matter would not. By measuring gravity, they were able to detect what looked like two clouds of dark matter, with a cloud of normal matter (hot gas) in between them.

In 2006, a group of scientists claimed that they had found a way to find dark matter.<ref name="stanford">{{cite journal|title=Dark matter observed|url=http://home.slac.stanford.edu/pressreleases/2006/20060821.htm}}</ref> Since dark matter is supposedly very different from normal matter, it is expected to act differently. The scientists observed two far-away [[galaxy cluster]]s that had crashed into each other at high speed: normal matter would have been scattered nearby after the collision, while dark matter would not. By measuring gravity, they were able to detect what looked like two clouds of dark matter, with a cloud of normal matter (hot gas) in between them.

File:Gravitational lens-full.jpg|Gravitational lensing bends light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.

File:Gravitational lens-full.jpg|Gravitational lensing bends light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.

File:Rotation_Curve_UGC11455.svg|The rotation curve of spiral galaxy UCG11455. The observed rotation for spiral galaxy UCG11455 is shown as points. The expected rotation from normal matter is shown in the line below.

File:Rotation_Curve_UGC11455.svg|The rotation curve of spiral galaxy [[UGC 11455]]. The observed rotation is shown as points with error bars. The expected rotation from normal matter is shown in the line below.

File:A slice through the Universe.webm|The positions in space of the galaxies identified by the VIPERS survey.

File:A slice through the Universe.webm|The positions in space of the galaxies identified by the VIPERS survey.

File:Dark matter map of KiDS survey region (region G12).jpg|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree Survey

File:Dark matter map of KiDS survey region (region G12).jpg|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree Survey

File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.webm|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]] observations of dwarf galaxies provide new insights on dark matter.

File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.webm|[[Fermi Gamma-ray Space Telescope|Fermi-LAT]] observations of dwarf galaxies provide new insights on dark matter.