Cosmic void

A cosmic void is a vast region of space with substantially lower matter density than the cosmic average. It is part of the large-scale cosmic web, often bounded by denser filaments and galaxy clusters.

Cosmic web: The large-scale network of galaxies, filaments, sheets, and voids formed by gravity in the expanding universe. Voids are the underdense regions within this network.
Matter density: The amount of matter per unit volume in a physical region. A void is defined by its matter density being well below the cosmic average.
Void finder: An algorithm that identifies underdense regions in a galaxy or matter distribution. Different methods assign void boundaries and sizes differently.
Galaxy cluster: A gravitationally bound system containing hundreds or thousands of galaxies and large amounts of hot gas and dark matter. Clusters are among the densest large-scale environments, opposite to void interiors.
Gravitational instability: The growth of density differences as gravity amplifies small variations in the distribution of matter. It drives matter out of underdense regions and into denser structures.
Cosmological principle: The assumption that the universe is statistically homogeneous and isotropic on sufficiently large scales. It supplies the cosmic average against which void underdensities are judged.
Void-galaxy cross-correlation: A statistical measure of how galaxies are distributed around cosmic voids. It probes void boundaries and the transition from underdensity to surrounding structure.
Cosmic filament: An elongated concentration of galaxies and matter connecting denser nodes in the cosmic web. Filaments mark the matter-rich boundaries and connections surrounding many voids.
Large-scale structure formation: The development of galaxies, clusters, filaments, and voids from early-universe density fluctuations. It explains how initially small differences produced the present-day void population.
Redshift: The increase in wavelength of light as an object recedes or as space expands. Astronomers use redshifts to map galaxies and identify underdense regions.

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