Editing Galaxy (section)

=== Modern research ===
[[File:Rotation curve of spiral galaxy Messier 33 (Triangulum).png|thumb|upright=1.8|Rotation curve of spiral galaxy [[Triangulum Galaxy|Messier 33]] (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a [[dark matter halo]] surrounding the galaxy.<ref>{{cite journal |last1=Corbelli |first1=E. |last2=Salucci |first2=P. |year=2000 |title=The extended rotation curve and the dark matter halo of M33 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=311 |issue=2 |pages=441–447 |arxiv=astro-ph/9909252 |bibcode=2000MNRAS.311..441C |bibcode-access=free |doi=10.1046/j.1365-8711.2000.03075.x |doi-access=free }}</ref>]]
In 1944, [[Hendrik C. van de Hulst|Hendrik van de Hulst]] predicted that [[microwave]] radiation with [[hydrogen line|wavelength of 21 cm]] would be detectable from interstellar atomic [[hydrogen]] gas;<ref>{{cite web
 |last1=Tenn
 |first1=Joe
 |title=Hendrik Christoffel van de Hulst
 |url=http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/
 |website=The Bruce Medalists
 |publisher=[[Sonoma State University]]
 |access-date=January 5, 2007
 |archive-date=January 14, 2012
 |archive-url=https://web.archive.org/web/20120114042216/http://www.phys-astro.sonoma.edu/BruceMedalists/vandeHulst/
 |url-status=dead
 }}</ref> and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of a rotating [[barred spiral galaxy|bar structure]] in the center of this galaxy.<ref>{{cite journal
 |last1=López-Corredoira |first1=M.
 |s2cid=18399375
 |display-authors=etal
 |date=2001
 |title=Searching for the in-plane Galactic bar and ring in DENIS
 |journal=[[Astronomy & Astrophysics]]
 |volume=373
 |issue=1 |pages=139–152
 |bibcode=2001A&A...373..139L
 |doi=10.1051/0004-6361:20010560
|arxiv = astro-ph/0104307 }}</ref> With improved [[radio telescope]]s, hydrogen gas could also be traced in other galaxies.
In the 1970s, [[Vera Rubin]] uncovered a discrepancy between observed galactic [[galaxy rotation curve|rotation speed]] and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen [[dark matter]].<ref>{{cite magazine
 |last=Rubin |first=Vera C.
 |author-link=Vera Rubin
 |date=1983
 |title=Dark matter in spiral galaxies
 |magazine=[[Scientific American]]
 |volume=248
 |issue=6 |pages=96–106
 |bibcode=1983SciAm.248f..96R
 |doi=10.1038/scientificamerican0683-96
}}</ref><ref>{{cite journal
 |last=Rubin |first=Vera C.
 |author-link=Vera Rubin
 |date=2000
 |title=One Hundred Years of Rotating Galaxies
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=112 |issue=772 |pages=747–750
 |bibcode=2000PASP..112..747R
 |doi=10.1086/316573
|s2cid=122927800
 |doi-access=free
 }}</ref>

Beginning in the 1990s, the [[Hubble Space Telescope]] yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars.<ref>{{cite press release
 |title=Hubble Rules Out a Leading Explanation for Dark Matter
 |website=Hubble News Desk
 |publisher=[[NASA]]
 |date=October 17, 1994
 |url=http://hubblesite.org/newscenter/archive/releases/1994/41/text/
 |access-date=January 8, 2007
 |archive-date=November 27, 2012
 |archive-url=https://web.archive.org/web/20121127083251/http://hubblesite.org/newscenter/archive/releases/1994/41/text/
 |url-status=dead
 }}</ref> The [[Hubble Deep Field]], an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ({{val|1.25|e=11}}) galaxies in the observable universe.<ref>{{cite web
 |last=Mattson
 |first=Barbara
 |editor-last=Gibb
 |editor-first=Meredith
 |date=November 27, 2002
 |title=How many galaxies are there?
 |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html
 |website=Imagine the Universe!
 |publisher=[[NASA]]
 |access-date=January 8, 2007
 |archive-date=July 28, 2012
 |archive-url=https://web.archive.org/web/20120728035236/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/021127a.html
 |url-status=dead
 }}</ref> Improved technology in detecting the [[electromagnetic spectrum|spectra]] invisible to humans (radio telescopes, infrared cameras, and [[x-ray astronomy|x-ray telescopes]]) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the [[Zone of Avoidance]] (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.<ref>{{cite journal
 |last1=Kraan-Korteweg |first1=R. C.
 |last2=Juraszek |first2=S.
 |s2cid=17900483
 |date=2000
 |title=Mapping the hidden Universe: The galaxy distribution in the Zone of Avoidance
 |journal=[[Publications of the Astronomical Society of Australia]]
 |volume=17 |issue=1 |pages=6–12
 |bibcode=2000PASA...17....6K
|arxiv = astro-ph/9910572
 |doi=10.1071/AS00006 }}</ref>

A 2016 study published in ''[[The Astrophysical Journal]],'' led by [[Christopher Conselice]] of the [[University of Nottingham]], analyzed many sources of data to estimate that the observable universe (up to z=8) contained at least two trillion ({{val|2|e=12}}) galaxies, a factor of 10 more than are directly observed in [[Hubble Space Telescope|Hubble]] images.<ref name="Conselice">{{cite journal|title=The Evolution of Galaxy Number Density at z <{{nbsp}}8 and its Implications|first=Christopher J.|last=Conselice|s2cid=17424588|display-authors=etal|journal=[[The Astrophysical Journal]]|volume=830|issue=2|year=2016|arxiv=1607.03909|bibcode= 2016ApJ...830...83C|doi=10.3847/0004-637X/830/2/83|page=83 |doi-access=free }}</ref>{{rp|12}}<ref name="NYT-20161017">{{cite news |last=Fountain |first=Henry |date=17 October 2016 |title=Two Trillion Galaxies, at the Very Least |work=[[The New York Times]] |url-access=registration |url=https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |url-status=live |access-date=17 October 2016 |archive-url=https://web.archive.org/web/20191231233343/https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |archive-date=December 31, 2019}}</ref> However, later observations with the [[New Horizons]] space probe from outside the [[zodiacal light]] observed less cosmic optical light than Conselice while still suggesting that direct observations are missing galaxies.<ref name="Lauer"/><ref>{{cite news |title=New Horizons spacecraft answers the question: How dark is space? |work=[[Phys.org]] |url=https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |access-date=15 January 2021 |language=en |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115110710/https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |url-status=live }}</ref>