1 Definition and terminology

1.1 Basic meaning of a void

In cosmology, a void is a region of space that contains far less matter than average. The term usually refers to a large underdense volume in the universe’s large-scale structure, where galaxies are sparse and the density of dark matter and ordinary matter is low relative to nearby regions. Voids are not completely empty; rather, they are characterized by a pronounced shortage of visible and invisible matter compared with the cosmic mean.

1.2 Void versus nearby large-scale structures

Voids are defined in relation to surrounding structures. They are commonly bordered by dense networks of galaxies, clusters, filaments, and walls. These neighboring formations shape the boundaries of voids and provide the contrast needed to identify them. In practice, a void is understood as part of a connected cosmic pattern rather than as an isolated empty region.

1.3 Common scientific uses of the term

The word void can be used more broadly in physics and astronomy to describe any region with unusually low density. In cosmology, however, it has a more specific meaning tied to large-scale structure. Scientists may also use related terms such as underdensity, underdense region, or cosmic void depending on context. The exact definition can vary between studies, especially when different methods are used to detect and measure these regions.

2 Voids in cosmology

2.1 Large-scale structure of the universe

Voids are a major component of the universe’s large-scale arrangement of matter. Together with filaments, clusters, and walls, they form the cosmic web, a vast network that describes how galaxies are distributed on the largest scales. Although they contain few galaxies, voids occupy a large fraction of cosmic volume.

2.1.1 Cosmic web context

The cosmic web is the pattern created by gravity acting on small early density differences. Dense regions evolve into galaxy clusters and filaments, while sparse regions become voids. This web-like arrangement is visible in galaxy surveys and simulations, which show a universe organized into interconnected dense threads surrounding wide low-density spaces.

2.1.2 Filaments, clusters, and walls

Filaments are elongated structures that connect dense nodes, while galaxy clusters are compact concentrations of many galaxies. Walls are broad sheet-like formations that can help enclose voids. These structures define the edges of voids and are often where most of the observable matter in a region is concentrated.

2.2 Formation of cosmic voids

Cosmic voids form from slight underdensities present in the early universe. Over time, gravity amplifies differences in matter distribution, causing denser areas to attract more matter and less dense areas to lose material to their surroundings. As a result, voids expand and become more pronounced as the universe evolves.

2.2.1 Density fluctuations in the early universe

The early universe was nearly uniform, but small fluctuations in density existed throughout it. Some regions were slightly denser, while others were slightly more diffuse. These tiny variations served as the starting point for later structure formation. The underdense patches eventually became the seeds of cosmic voids.

2.2.2 Gravitational expansion of underdense regions

Underdense regions experience weaker gravitational pull than their surroundings. As nearby denser regions attract matter, voids tend to lose material and expand relative to the rest of the universe. This expansion is not the same as the overall cosmic expansion, but it is enhanced by the local lack of mass.

2.3 Properties of cosmic voids

Voids differ widely in shape, size, and internal structure. They may contain a few galaxies, diffuse gas, and dark matter, but at much lower densities than average. Their physical characteristics depend on how they formed and on the surrounding matter distribution.

2.3.1 Size and shape

Cosmic voids range from tens to hundreds of millions of light-years across. Their outlines are often irregular rather than perfectly spherical, reflecting the complex geometry of the surrounding web. Some are relatively isolated, while others are adjacent or partially connected, forming larger underdense complexes.

2.3.2 Matter content

Although voids are sparse, they are not truly empty. They can contain faint galaxies, intergalactic gas, and dark matter spread at low density. The amount of matter inside a void is much smaller than in filaments or clusters, but the exact content depends on the void’s type and environment.

2.3.3 Expansion and evolution

Voids evolve over cosmic time by becoming larger and more empty relative to their surroundings. As matter flows outward, internal density decreases and boundary structures become more distinct. This evolution makes voids valuable for studying how structure grows in an expanding universe.

3 Observational study of voids

3.1 Galaxy surveys

Astronomers identify voids mainly through large galaxy surveys. These surveys map the positions and redshifts of galaxies across wide regions of the sky, revealing where matter is concentrated and where it is scarce. By comparing observed galaxy distributions with statistical expectations, researchers can locate candidate voids.

3.2 Mapping underdense regions

To map voids, scientists use catalogues of galaxies and apply geometric or density-based algorithms. These methods estimate the boundaries of low-density regions and measure their sizes and shapes. Some approaches focus on the distribution of galaxies, while others incorporate broader information about the underlying matter field.

3.3 Statistical measures

Void studies often rely on statistics to describe populations of underdense regions. Such measurements help compare different surveys, model predictions, and simulations. They also provide a way to summarize complex spatial patterns in the cosmic web.

3.3.1 Void size distribution

The void size distribution describes how many voids occur at different scales. Smaller voids are generally more numerous, while large voids are rarer. This distribution offers insight into the growth of cosmic structure and the physical processes that shape the universe.

3.3.2 Void abundance and volume fraction

Void abundance refers to the number of voids found in a given region or survey, while volume fraction indicates how much of the universe’s volume is occupied by them. Even though voids hold little mass, they can account for a substantial fraction of cosmic space.

3.4 Challenges in detection

Identifying voids is not always straightforward. Survey limits, incomplete data, and observational bias can affect results. Because void boundaries are not sharply defined, different detection methods may produce different void catalogues. In addition, faint galaxies and sparse sampling can make low-density regions difficult to distinguish from data gaps.

4 Theoretical models

4.1 Gravitational instability models

Many theories of void formation are based on gravitational instability, in which small initial fluctuations grow over time. In these models, slightly underdense regions expand relative to the background universe, while overdense regions collapse into denser structures. The contrast between these processes helps explain the emergence of the cosmic web.

4.2 Simulation-based approaches

Computer simulations are widely used to study voids because they can track the evolution of matter over billions of years. These models help scientists compare theoretical predictions with observed galaxy distributions and explore how voids respond to different cosmological assumptions.

4.2.1 N-body simulations

N-body simulations model the motion of large numbers of particles under gravity. They are especially useful for studying dark matter and the formation of large-scale structure. In such simulations, voids appear naturally as matter collects into dense filaments and leaves sparse regions behind.

4.2.2 Hydrodynamic simulations

Hydrodynamic simulations add the behavior of gas to gravitational models. They can show how temperature, pressure, and feedback processes influence matter inside and around voids. These calculations are more complex than N-body methods but provide a richer picture of cosmic environments.

4.3 Analytical descriptions

Analytical models attempt to describe void formation with mathematical formulas rather than full numerical simulations. These approaches can capture broad trends, such as how voids expand or how their sizes are distributed. They are useful for interpreting observations and for building simplified theories of structure growth.

5 Physical significance

5.1 Tests of cosmological models

Voids provide a valuable way to test theories of the universe. Their abundance, shapes, and expansion behavior can be compared with predictions from cosmological models. Because voids respond to gravity on large scales, they can reveal whether a model accurately describes structure formation.

5.2 Constraints on dark matter and dark energy

Studies of voids can help constrain the properties of dark matter and dark energy. Dark matter influences how quickly structures form, while dark energy affects the rate of cosmic expansion. Measurements of void behavior therefore offer indirect information about the ingredients that govern the universe’s evolution.

5.3 Role in structure formation

Voids are not merely empty gaps; they are active parts of structure formation. Their growth affects the redistribution of matter and the arrangement of galaxies around them. By studying voids, researchers gain insight into the balance between collapse in dense regions and expansion in sparse ones.

6.1 Cosmic web

The cosmic web is the large-scale network of filaments, clusters, walls, and voids that describes matter distribution in the universe.

6.2 Galaxy clusters

Galaxy clusters are dense concentrations of galaxies and dark matter that often border voids and help define their limits.

6.3 Supervoids

Supervoids are exceptionally large voids that extend over especially great distances and may consist of multiple connected underdense regions.

6.4 Empty space in other scientific contexts

In other sciences, empty space may refer to vacuum conditions, low-pressure regions, or conceptual gaps, but these uses differ from the cosmological meaning of a void.