The Cosmic Web: Filaments, Voids, and Superclusters

The cosmic web describes the uneven distribution of matter across scales much larger than individual galaxies. Its filaments link dense concentrations, while less crowded regions open between them. The pattern reflects both the initial density field and gravitational tides: the unequal pulls exerted by surrounding matter. Its connections are physical concentrations of matter, not solid threads.[1]

A map can contain a prominent filament, smaller branches within it, and compact dark matter halos hosting galaxies. The web therefore has structure within structure. Changing the scale over which density is smoothed changes which features stand out; there is no single outline that every mapping method must recover.[2]

Start with a nearly uniform universe containing slightly denser and less dense regions. In the standard cold dark matter picture, gravity gradually amplifies those differences. For scales inside the horizon, growth is restricted during radiation domination; it is misleading to describe rapid clumping as simply beginning when dark matter thermally decouples. After matter comes to dominate, gravitational growth becomes more effective.[3]

Here, cold means that dark matter particles moved much more slowly than light during the early stages relevant to structure formation; it does not identify their nature. Cosmic expansion lowers the average density while sufficiently overdense regions can become increasingly concentrated relative to that average. Once the contrasts grow large, a simple linear description fails: flows intersect, halos form, and mergers reshape the distribution. Ordinary gas also responds to pressure, cooling, heating, and feedback, so it does not follow collisionless dark matter in every detail.[3]

Collapse need not proceed equally in every direction

A region feels different gravitational pulls along different directions. Compression chiefly along one direction can produce a flattened structure; compression along two can produce a filament. Strong collapse in all directions helps build dense knots. This is a useful physical picture, rather than a compulsory sequence through which every object passes. The surrounding tidal field helps determine which peaks become connected.[1]

The quieter regions have histories of their own

A void has a lower matter density than its surroundings. It can still contain dark matter, diffuse gas, isolated galaxies, and faint internal filaments. Voids evolve as matter flows toward denser surroundings; they can merge, while some smaller underdense regions are squeezed by the collapse of their larger environment. Empty-looking space is part of the dynamics.[4]

There is no universal percentage of the universe that every method must classify as void. Researchers can identify voids from galaxies, reconstructed density, or other fields, using different boundaries and thresholds. Reported sizes, volume fractions, and galaxy counts must therefore be interpreted alongside the definition and survey selection.[5]

A supercluster is not automatically one bound object

Traditional superclusters are large associations of groups and clusters. Many are not in dynamical equilibrium, or virialized. That is a different question from whether a region is gravitationally bound or will eventually collapse. Definitions based on present-day connectedness, velocity flows, and future collapse can select different boundaries. A striking map alone cannot settle the region’s long-term fate.[6]

An influential milestone was the 1986 CfA redshift slice published by Valérie de Lapparent, Margaret Geller, and John Huchra. Its galaxy distribution made connected concentrations and large underdense regions especially vivid. It built on earlier discoveries of clustering and voids; the cosmic web did not emerge from a single observation.[9]

The 2dF Galaxy Redshift Survey expanded spectroscopic mapping using an instrument that observed many targets together. The Sloan Digital Sky Survey combined systematic imaging with spectroscopy. Such projects made the selection of galaxies, measurement quality, and survey completeness central parts of the scientific analysis.[10], [11]

A redshift is a measurement, not a distance label

A galaxy’s spectrum reveals how much familiar spectral features have shifted in wavelength. Astronomers combine this redshift with its sky position and an assumed expansion history to estimate a three-dimensional location. Different distance measures answer different questions, and a distant galaxy is observed at an earlier stage of cosmic history.[12]

Galaxies also have peculiar velocities: motions relative to the average expansion. Those motions affect observed redshifts, stretching some clusters along the inferred line of sight and altering larger-scale clustering patterns. These redshift-space distortions require modeling, but also provide information about gravitational growth.[13]

Finally, bright, faint, red, and blue galaxies do not all cluster in the same way. A map’s appearance depends partly on which population the survey can detect and chooses to observe. Correcting this relationship between selected galaxies and underlying matter—often called galaxy bias—is essential for comparing surveys with theory.[14]

Gravitational lensing: weighing the structure

Gravity changes the paths of background light. In weak lensing, researchers measure subtle statistical distortions in many galaxy shapes to infer foreground mass projected along sightlines. The result is sensitive to dark matter as well as ordinary matter. Source distances, intrinsic shapes, measurement noise, and reconstruction ambiguities all affect the inference.[15]

One example is the filament between Abell 222 and Abell 223. A 2012 study reported weak-lensing evidence for the connecting mass concentration, alongside an excess of galaxies and diffuse X-ray emission. Agreement between those observations strengthens the physical interpretation; the lensing map is not a photograph of individual dark matter particles.[17]

Intergalactic gas: finding the faint ordinary matter

Much ordinary matter lies outside stars. Some filament gas is detectable through X-ray emission, while absorption features in background light probe gas under other conditions. A 2025 study combined Suzaku and XMM-Newton observations to examine hot gas in a Shapley supercluster filament, carefully accounting for neighboring clusters and point sources. Such work measures particular gas components rather than every filament at once.[16]

Another tool is the thermal Sunyaev–Zel’dovich effect. Energetic electrons transfer energy to some passing CMB photons, slightly changing the background radiation’s spectrum. This reveals ionized gas through its interaction with ancient light, even when the gas is difficult to study by its own emission.[18]

Because individual filament signals can be extremely faint, researchers sometimes stack observations of many similar galaxy pairs. A 2019 analysis used this approach with Planck data to investigate a statistical thermal signal between galaxies. Interpreting the average still requires modeling gas associated with galaxy halos and other contaminants.[19]

A simulation begins with specified initial fluctuations and cosmological assumptions, then follows how matter evolves. In an N-body calculation, numerical particles represent parcels of mass, not individual fundamental particles. The original Millennium Simulation tracked collisionless matter gravitationally; galaxy properties were subsequently added through semi-analytic models based on the resulting halo histories.[20]

Hydrodynamic simulations also evolve gas. Projects such as EAGLE include cooling, star formation, enrichment, and feedback through a combination of resolved calculations and prescriptions for processes below the simulation’s resolution. These subgrid models are part of the physical assumptions, not simply a way to draw more detailed galaxies.[21]

Finite volume limits which large structures fit inside a calculation; finite resolution limits the smaller processes it follows. Some model parameters are calibrated against observations. Matching the same quantities used for calibration is therefore not an independent prediction. Stronger tests compare the model with additional observations and check how results change with resolution or modeling choices.[21]

The most informative comparison goes beyond whether two pictures look alike. Researchers compare statistical patterns, selected galaxy populations, and observables calculated as they would be measured in a real survey.

Environment can alter a galaxy’s supply of gas. When a galaxy moves through sufficiently dense surrounding gas, ram pressure can remove some of its interstellar material. In clusters and some groups, this can reduce the fuel available for new stars. The outcome depends on the surrounding density, the galaxy’s speed, and how strongly its own gravity holds the gas.[22]

But proximity to a filament does not by itself prove what caused a galaxy’s color or gas content. Filaments also contain groups, and galaxy properties depend on stellar mass and host-halo mass. A 2024 analysis of the RESOLVE and ECO surveys found that many filament-related trends were largely explained by the increasing importance of groups near filaments, with additional effects requiring a more careful separation.[23]

This turns “Does the web influence galaxies?” into a sharper research question: which property changes, compared with what sample, and after accounting for which other environmental factors? A single label such as “filament galaxy” cannot capture an entire evolutionary history.

The apparent distortions caused by galaxy motions connect clustering to the rate at which structure grows. Their interpretation also depends on how galaxies trace mass. Jointly analyzing these effects makes the web a test of gravitational growth, rather than merely a catalog of interesting shapes.[13]

Voids provide another view of the density field. Their abundance, apparent shapes, and surrounding matter profiles can carry information about expansion and gravitational growth.[24] Consistent definitions matter here: changing the void finder or tracer population can change the sample being compared with a theoretical prediction.[5]

Small scales test more than dark matter alone

A predicted dark matter halo is not automatically an observable galaxy. Gas heating, stellar feedback, and the ability of a small halo to retain gas affect which systems become luminous. Reviews of dwarf-galaxy modeling show that including ordinary-matter physics changes several apparent tensions with dark-matter-only predictions. Questions about internal density profiles and the diversity of dwarf galaxies remain useful tests of the combined model.[25]

The standard framework earns support through successful quantitative comparisons. A web-like pattern by itself does not uniquely establish the identity of dark matter, the origin of the initial fluctuations, or the explanation for accelerated expansion.

Survey status: 8 September 2026. Observing programs and release schedules evolve; the stages below distinguish ongoing surveys from preparations for science operations.

NASA’s Nancy Grace Roman Space Telescope is in commissioning, traveling toward the Sun–Earth L2 region while its systems are activated, adjusted, and calibrated. As of this article’s status date, it should be described as preparing for science operations, rather than already delivering its planned cosmological survey.[28]

Mapping collective light

Line-intensity mapping measures combined spectral-line emission across direction and frequency, often including galaxies too faint to identify individually. Neutral hydrogen’s 21-centimeter line and molecular emission such as carbon monoxide offer different tracers. The measured intensity depends on gas and emission physics as well as the distribution of matter.[29]

Comparisons with galaxy surveys and other signals can help isolate the cosmological pattern. Foregrounds, overlapping emission lines, instrumental effects, and the relationship between luminosity and mass still require careful treatment. A larger map becomes more powerful when those limitations are understood.[29]

Cosmology and the Universe’s Large-Scale Structure

1. Cosmic Inflation: Theory and Evidence
2. The Cosmic Web: Filaments, Voids, and Superclusters · You are here
3. The Cosmic Microwave Background’s Detailed Structure
4. Baryon Acoustic Oscillations
5. Redshift Surveys and Mapping the Universe
6. Gravitational Lensing: A Natural Cosmic Telescope
7. Measuring the Hubble Constant: The Tension
8. Dark Energy Surveys
9. Anisotropies and Inhomogeneities
10. Current Debates and Outstanding Questions