Galaxies do not spread evenly through the universe. They clump into groups and clusters, and clusters clump into superclusters. A supercluster is one of the largest structures in the universe, a vast collection of galaxy clusters bound together by gravity. The Milky Way is part of the Laniakea Supercluster, which contains about 100,000 galaxies and spans 500 million light-years.

Superclusters are not gravitationally bound in the same way as clusters. They are expanding with the universe, and their member clusters will eventually drift apart. They are held together loosely by dark matter and the gravitational influence of nearby superclusters. The boundaries between superclusters are not sharp. They are defined by the flow of galaxies toward gravitational centers.

The largest known supercluster is the Hercules-Corona Borealis Great Wall, which spans about 10 billion light-years. Structures that large are difficult to explain with current models of cosmology, which predict that the universe should be more uniform at the largest scales. The discrepancy is an active area of research.

Superclusters are useful for studying dark matter and dark energy. Their distribution and motion reveal how gravity and expansion compete on the largest scales. They are the cosmic web's biggest knots.