Void (astronomy)

Characteristics

Voids typically contain few or no luminous galaxies; the galaxy population inside them is sparse, and those galaxies that do exist tend to be small and gas-rich rather than massive ellipticals. The physical diameter of common voids ranges widely, with many falling in a scale of roughly 11 to 150 Mpc. Larger empty regions that exclude many superclusters are often called supervoids. Although underdense in visible matter, voids still host dark matter, diffuse gas, and tenuous filamentary structures on smaller scales.

Formation and evolution

Cosmic voids arise from the growth of initial density fluctuations in the early Universe. Slight underdensities expand faster than their surroundings under gravity and become increasingly emptier as matter flows toward overdense filaments and clusters. Over time voids can merge and reshape one another, producing complex, sponge-like patterns. Their internal dynamics and shape are influenced by surrounding structures, the cosmic expansion rate and the properties of dark matter and dark energy.

Observation and significance

Voids are revealed by large redshift surveys and mapped by catalogues derived from them. Systematic studies of voids use galaxy surveys and statistical measures of clustering to quantify their sizes, shapes and frequency. Observational work — from early surveys to modern projects — uses void statistics to test cosmological models, probe dark energy, and constrain how galaxy formation depends on environment. A well-known historical catalogue and counting of large voids was published in the 1990s; for an early compilation see the 1994 counting.

Notable examples and distinctions

• Local Void: an underdense region near the Local Group that affects nearby galaxy motions and peculiar velocities.
• Boötes Void: a famous large, relatively empty region discovered in the late 20th century and often cited in popular descriptions of cosmic emptiness.
• Eridanus/Cold Spot supervoids: proposed very large underdensities that have been discussed in relation to features in the cosmic microwave background.

Voids contrast with galaxy-rich structures: filaments, walls and clusters host most of the luminous matter while voids represent most of the Universe’s volume. They are therefore important laboratories for understanding cosmology and galaxy evolution in low-density environments. Observational efforts continue to refine void catalogues using modern surveys and to compare void properties with theoretical predictions from simulations and models of structure formation (galaxy studies and survey analyses often reference these comparisons).

For readers seeking detailed maps and statistical treatments, many contemporary studies and public data releases from major surveys provide void catalogues and visualizations; these resources are often linked from survey pages and review articles (survey portals, cosmology overviews, and project documentation). The observational measurement unit for void sizes — the megaparsec — is commonly used in this literature (Mpc references).

Because supervoids sit between regions of high concentration such as clusters and superclusters, they can influence light propagation and the integrated Sachs–Wolfe effect in the cosmic microwave background; these subtle signatures make voids a continuing focus of cosmological inquiry and cross-checks between theory and observation. Historical compilations of large voids and lists of examples remain useful starting points for this work (early lists).

Researchers and interested readers can find further background in reviews and survey documentation, which summarize methods for void identification, cataloguing and the role of voids in testing cosmological models. For practical searches and transcriptions of void data, consult project pages and archived catalogues (size and scale notes, galaxy catalogues).