A supercluster is a large group of smaller galaxy clusters or galaxy groups. it is among the largest-known structures of the cosmos. The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies), which in turn is part of the Laniakea Supercluster.This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years.The number of superclusters in the observable universe is estimated to be 10 million.
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.
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