Editing Supercluster (section)

==Existence==
The existence of superclusters indicates that the galaxies in the Universe are not uniformly distributed; most of them are drawn together in groups and clusters, with groups containing up to some dozens of galaxies and clusters up to several thousand galaxies. Those groups and clusters and additional isolated galaxies in turn form even larger structures called superclusters.[[File:An Intergalactic Heavyweight.jpg|thumb|left|The Abell 901/902 supercluster is located a little over two billion light-years from Earth.<ref>{{cite news|title=An Intergalactic Heavyweight|url=http://www.eso.org/public/images/potw1304a/|access-date=12 February 2013|newspaper=ESO Picture of the Week}}</ref>|200x200px]]

Their existence was first postulated by [[George O. Abell|George Abell]] in his 1958 [[Abell catalogue]] of galaxy clusters. He called them "second-order clusters", or clusters of clusters.<ref>{{cite journal |author=Abell |first=George O. |date=1958 |title=The distribution of rich clusters of galaxies. A catalogue of 2,712 rich clusters found on the National Geographic Society Palomar Observatory Sky Survey |url=https://thesis.library.caltech.edu/1581/1/Abell_go_1957.pdf |journal=[[The Astrophysical Journal Supplement Series]] |volume=3 |pages=211–288 |bibcode=1958ApJS....3..211A |doi=10.1086/190036}}</ref>

Superclusters form massive structures of galaxies, called [[galaxy filament|"filaments"]], "supercluster complexes", "walls" or "sheets", that may span between several hundred million light-years to 10 billion light-years, covering more than 5% of the [[observable universe]]. These are the largest structures known to date. Observations of superclusters can give information about the initial condition of the universe, when these superclusters were created. The directions of the rotational axes of galaxies within superclusters are studied by those who believe that they may give insight and information into the early formation process of galaxies in the history of the Universe.<ref name="Huetal2006">
{{cite journal
 | author=Hu, F. X.
 | display-authors=etal
 | date=2006
 | title=Orientation of Galaxies in the Local Supercluster: A Review
 | journal=[[Astrophysics and Space Science]]
 | volume=302 | issue=1–4 | pages=43–59
 | arxiv=astro-ph/0508669
 | bibcode=2006Ap&SS.302...43H
 | doi=10.1007/s10509-005-9006-7
| s2cid=18837475
 }}</ref>
 
Interspersed among superclusters are large [[void (astronomy)|voids]] of space where few galaxies exist. Superclusters are frequently subdivided into groups of clusters called [[galaxy groups and clusters]].

Although superclusters are supposed to be the largest structures in the universe according to the [[Cosmological principle]], larger structures have been observed in surveys, including the [[Sloan Great Wall]].<ref>{{Cite journal|last1=Nurmi|first1=P.|last2=Heinamaki|first2=P.|last3=Martinez|first3=V. J.|last4=Einasto|first4=J.|last5=Enkvist|first5=I.|last6=Einasto|first6=P.|last7=Tago|first7=E.|last8=Saar|first8=E.|last9=Tempel|first9=E.|date=2011-05-09|title=The Sloan Great Wall. Morphology and galaxy content|journal=[[The Astrophysical Journal]]|volume=736|issue=1|pages=51|language=en|doi=10.1088/0004-637X/736/1/51|arxiv=1105.1632|bibcode=2011ApJ...736...51E|s2cid=119215944}}</ref>