The Abell 901/902 supercluster is located a little over two billion light-years from Earth.
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.
Their existence was first postulated by George Abell in his 1958 Abell catalogue of galaxy clusters. He called them "second-order clusters", or clusters of clusters.
Superclusters form massive structures of galaxies, called "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 known structures 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 may also give insight and information into the early formation process of galaxies in the history of the Universe.
Interspersed among superclusters are large 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.
It contains the Local Group with our galaxy, the Milky Way. It also contains the Virgo Cluster near its center, and is sometimes called the Local Supercluster. It is thought to contain over 47,000 galaxies.
In 2014, the newly announced Laniakea Supercluster subsumed the Virgo Supercluster, which became a component of the new supercluster.
Discovered in 1999 (as ClG J0848+4453, a name now used to describe the western cluster, with ClG J0849+4452 being the eastern one), it contains at least two clusters RXJ 0848.9+4452 (z=1.26) and RXJ 0848.6+4453 (z=1.27) . At the time of discovery, it became the most distant known supercluster. Additionally, seven smaller groups of galaxies are associated with the supercluster.
A rich supercluster with several galaxy clusters was discovered around an unusual concentration of 23 QSOs at z=1.1 in 2001. The size of the complex of clusters may indicate a wall of galaxies exists there, instead of a single supercluster. The size discovered approaches the size of the CfA2 Great Wall filament. At the time of the discovery, it was the largest and most distant supercluster beyond z=0.5 
This supercluster at the time of its discovery was the largest supercluster found so deep into space, in 2000. It consisted of two known rich clusters and one newly discovered cluster as a result of the study that discovered it. The then known clusters were Cl 1604+4304 (z=0.897) and Cl 1604+4321 (z=0.924), which then known to have 21 and 42 known galaxies respectively. The then newly discovered cluster was located at 16h 04m 25.7s, +43° 14′ 44.7″
^Cucciati, O.; Lemaux, B. C.; Zamorani, G.; Le Fevre, O.; Tasca, L. A. M.; Hathi, N. P.; Lee, K-G.; Bardelli, S.; Cassata, P.; Garilli, B.; Le Brun, V.; Maccagni, D.; Pentericci, L.; Thomas, R.; Vanzella, E.; Zucca, E.; Lubin, L. M.; Amorin, R.; Cassara', L. P.; Cimatti, A.; Talia, M.; Vergani, D.; Koekemoer, A.; Pforr, J.; Salvato, M. (2018). "The progeny of a Cosmic Titan: a massive multi-component proto-supercluster in formation at z=2.45 in VUDS". arXiv:1806.06073 [astro-ph.CO].
Tanaka, I. (2004). "Subaru Observation of a Supercluster of Galaxies and QSOS at Z = 1.1". Studies of Galaxies in the Young Universe with New Generation Telescope, Proceedings of Japan-German Seminar, held in Sendai, Japan, July 24–28, 2001. pp. 61–64. Bibcode:2004sgyu.conf...61T.