Supercluster

Superclusters are vast cosmic structures consisting of numerous galaxy clusters and galaxy groups bound together in large-scale associations. They rank among the largest known features in the observable universe and illustrate the hierarchical nature of cosmic structure formation. The Milky Way resides within the Local Group, which is itself part of the Virgo Supercluster. This supercluster is further embedded in the larger Laniakea Supercluster, which in turn forms part of the extensive Pisces–Cetus Supercluster Complex. Because of their extremely low densities on cosmic scales, superclusters do not remain gravitationally bound systems and instead participate in the general expansion of the universe governed by the Hubble flow. Current estimates suggest that roughly ten million superclusters may exist within the observable universe.

Large-scale distribution and cosmic significance

The existence of superclusters confirms that galaxies are not uniformly distributed across cosmic space. Instead, most galaxies occur in gravitationally bound groups containing a few dozen members or clusters containing several thousand galaxies. These smaller systems often gather together into superclusters, which may extend hundreds of millions to billions of light years. Early recognition of such structures came from the work of George O. Abell, who in 1958 postulated their presence while compiling his catalogue of galaxy clusters, describing them as “second-order clusters” or clusters of clusters.
Superclusters are major components of the cosmic web, a network of galaxy filaments, walls, and sheets that outline the large-scale structure of the universe. Some supercluster complexes reach lengths of several billion light years and cover more than five per cent of the observable cosmos. Their immense size makes them valuable probes of early conditions in the universe. Observations of galaxy orientations within superclusters, for instance, offer insight into the processes that influenced galaxy formation and rotation in the early cosmological environment. Between these large superclusters lie vast cosmic voids containing comparatively few galaxies.
Although superclusters have long been regarded as the largest coherent structures in the universe, surveys continue to reveal even greater concentrations of matter. Discoveries such as the Sloan Great Wall challenged earlier conceptions of cosmic scale and highlighted the complexity of large-scale structure.

Supercluster characteristics and classification

Superclusters are composed of multiple galaxy groups and clusters linked across immense distances. Their masses are typically expressed in solar masses, and their spatial extents are measured in hundreds of millions of light years. They are often subdivided into regional components or cluster chains and may form part of yet larger supercluster complexes.
One of the most prominent examples is the Virgo Supercluster, historically recognised as the home structure of the Local Group. In 2014, however, studies showed that Virgo constitutes only one lobe of a larger structure known as the Laniakea Supercluster. Laniakea integrates the previously distinct Hydra–Centaurus and Pavo–Indus Superclusters, revealing a unified system shaped by coherent gravitational flows. This reinterpretation enhanced understanding of local large-scale cosmic geography and altered the framework for mapping nearby supercluster boundaries.
Nearby superclusters can be relatively well characterised. Some exhibit dimensions on the order of hundreds of millions of light years, such as systems extending 360 million light years and containing masses measured in multiples of 10¹⁶ solar masses. Others include well-studied components such as the Fornax, Dorado, and Eridanus clusters, which trace distinct substructures within larger associations.
More distant superclusters are generally identified by redshift, with values that indicate their remoteness and the expansion of the universe. Superclusters at redshifts around 0.06–0.12 lie hundreds of millions to over a billion light years away. Extremely distant examples, such as structures discovered at redshifts approaching 1.3, represent superclusters as they appeared when the universe was only a fraction of its current age. These systems may contain multiple forming clusters, along with several groups of galaxies still assembling under gravity.

Large-scale structures and cosmic extremes

Some superclusters challenge conventional expectations regarding the scale of cosmic structure. Observations at high redshift have identified superclusters spanning tens of megaparsecs, containing clusters and groups that are still in the process of coalescing. These structures, sometimes labelled by their catalogue designations, often include multiple massive galaxy clusters with redshifts exceeding 1.2, making them among the most distant supercluster systems known.
At lower redshifts, surveys have catalogued systems spanning several hundred million light years and displaying complex internal structure. Their physical characteristics vary widely, with lengths ranging from around 110 million to over 650 million light years. Some recently identified superclusters occupy only small regions of the sky but reveal dense concentrations of clusters, while others form components of larger cosmic walls and filaments.

Superclusters in the cosmic web

Superclusters exist within an interconnected lattice of filaments, nodes, and voids, collectively known as the cosmic web. Galaxies trace these structures through their distribution patterns, while gravitational interactions shape the evolution of clusters and groups within superclusters. Despite their immense size, superclusters do not form gravitationally bound entities; instead, their components gradually separate due to cosmic expansion. Their internal motions, cluster distributions, and filamentary connections reveal key information about dark matter, dark energy, and the growth of structure over cosmic time.