Galaxy Clusters and the Cosmic Web
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 02 / Article 08
Galaxy clusters & the cosmic web
Beyond individual galaxies lies a changing network of matter: dense clusters, connecting filaments, broad sheets, and vast underdense regions.
Galaxies belong to a much larger pattern.
Most individual stars visible in the night sky are part of our own Galaxy. To discover the structure beyond it, astronomers map entire populations of galaxies across enormous distances.
Those galaxies form a striking pattern of concentrations and connections, separated by regions with far fewer members. The name cosmic web describes this arrangement of matter.[1], [2]
Its most prominent clusters are only one part of the story. Gas between galaxies, matter that emits no light, and the structure of the voids all help reveal how the universe has evolved.
What are nodes, filaments, sheets, and voids?
The web is a three-dimensional density pattern. Its features have irregular shapes and overlapping boundaries; astronomers use different methods to identify them in surveys and simulations.[2]
Nodes
Prominent nodes are places where several filaments meet. They can host massive galaxy groups and clusters, which continue acquiring matter from their surroundings.
Filaments
These concentrations of dark matter, gas, and galaxies link denser regions. They can guide flows toward massive halos while containing smaller structures of their own.
Sheets and walls
Matter also forms broad structures that are thin compared with their extent. They have finite thickness and can contain or connect smaller filaments.
Voids
Voids contain fewer galaxies and less matter per unit volume. Small galaxies, tenuous gas, dark matter, and finer internal structures can still be present.[3]
How does gravity create a web?
The early universe was nearly uniform, with small variations in density. Regions containing slightly more matter exerted a stronger gravitational pull and grew denser relative to their surroundings.
The pull was not identical in every direction. Matter could become compressed strongly along one direction while remaining extended along others. This helps explain why cosmic structure develops flattened sheets and elongated filaments, as well as compact halos.[1]
Compression mainly along one direction gives a sheetlike shape; along two, a filamentlike shape; along three, a compact concentration. This is a useful geometric picture of an evolving, complicated process.
Real structures grow together, exchange material, and merge. They do not all follow a fixed sequence from sheet to filament to cluster.[1], [2]
The pattern began before galaxies lit it up
Dark matter structure developed through the Dark Ages and cosmic dawn. As gas cooled and stars formed, galaxies became visible tracers of a much larger matter distribution.
The web continued evolving before, during, and after reionization. The changing balance between gravity, gas physics, and cosmic expansion shaped the structures observed at later times.[2]
Does dark energy create the voids?
Underdense regions can expand faster than their surroundings as gravity amplifies the density contrast. This behavior occurs even in models without dark energy.
Dark energy changes the universe’s late-time expansion and the growth of structure. It affects void evolution, but it is not the sole cause. The surroundings also matter: some small voids can be squeezed by a larger region that is collapsing.[3]
The earlier stages are explored in Gravitational Clumping and Density Fluctuations.
What is inside a galaxy cluster?
A galaxy cluster is a gravitationally bound system that may contain hundreds or thousands of galaxies within a shared halo. Clusters are among the largest collapsed, bound structures in the universe, and they continue growing through inflow and mergers.[17]
| Component | Its typical contribution | Where it is found |
|---|---|---|
| Dark matter | Roughly 85% of the total mass in a massive cluster | An extended distribution surrounding and overlapping its galaxies and gas |
| Hot gas | Most of the remaining mass | The space between galaxies, forming the intracluster medium |
| Stars | Only a few percent of the total mass | Inside galaxies and in a diffuse population of intracluster stars |
These are approximate proportions. They vary with cluster mass, measurement method, and the radius included in the inventory.[4]
Why is the gas so hot?
As gas falls into a cluster and encounters other gas, shocks convert part of its motion into heat. The intracluster medium, or ICM, commonly reaches around 10–100 million kelvin.
At these temperatures it is a dilute plasma of ions and free electrons. This is very different from the cool, dense gas clouds that directly form stars. Its temperature, motions, and radiation reveal how the cluster is growing.[5], [17]
How do astronomers map matter they cannot simply see?
A galaxy image, an X-ray map, and a gravitational-lensing reconstruction show different properties of the same region. Combining them allows researchers to compare the visible galaxies, the plasma, and the total gravitational mass.
| Method | What it reveals | What needs interpretation |
|---|---|---|
| Galaxy images and redshifts | Where luminous galaxies lie and how they cluster | Selection effects and galaxy motions relative to cosmic expansion |
| X-ray observations | Hot plasma, including information about its temperature and composition | Faint emission, gas density structure, and assumptions used to infer mass |
| Sunyaev–Zeldovich effect | The influence of energetic electrons on the cosmic microwave background | A pressure-related gas signal, whose connection to total mass must be calibrated |
| Gravitational lensing | The gravitational effect of ordinary matter and dark matter together | Projection, background-galaxy shapes and distances, and reconstruction uncertainties |
These techniques test one another’s assumptions and help identify disturbed or merging systems.[5], [6], [7], [10]
A cluster can leave a mark on ancient light
In the thermal Sunyaev–Zeldovich effect, photons from the Cosmic Microwave Background scatter from energetic electrons in cluster gas and gain energy on average. The result is a characteristic change in the microwave spectrum.
This provides a way to detect and study hot gas through its effect on background radiation, alongside measurements of the gas’s own X-ray emission.[10]
Is a lensing map a photograph of dark matter?
Lensing measures how gravity changes background light. Weak-lensing analyses combine small distortions in many galaxy shapes to reconstruct a weighted, integrated matter distribution along the line of sight.
The inferred mass includes ordinary matter as well as dark matter. Recovering depth or separating the components requires additional observations and modeling.[6]
Why can a cluster look stretched in a redshift map?
A galaxy’s observed redshift contains the effect of cosmic expansion and of its own motion relative to that expansion. Galaxies orbiting within a cluster can therefore appear spread out along the line of sight when redshift is used as a distance coordinate.
These redshift-space distortions must be modeled. They also carry useful information about gravitational motions.[7]
What is the “missing matter” between galaxies?
The missing-baryon problem concerns ordinary matter expected from early-universe measurements but difficult to account for in nearby stars and easily observed gas. It is separate from the question of what dark matter is.
Much of that ordinary matter is spread through diffuse, ionized gas. Some lies in filaments as the warm-hot intergalactic medium, or WHIM. Its low density makes it faint even when its temperature is high.[11], [13]
Fast radio bursts
Free electrons delay the lower-frequency parts of a radio pulse more than its higher-frequency parts. For a burst with a known host distance, that dispersion helps measure the intervening ionized material.
Macquart and colleagues used this approach in 2020 to obtain a cosmic baryon census consistent with early-universe estimates.[11]
From a census to a distribution
A 2025 analysis by Connor and colleagues used a larger sample of localized bursts to estimate how baryons are partitioned among intergalactic space and halos.
The results favored a large intergalactic reservoir. Such measurements constrain the gas inventory statistically; they do not supply a complete image of every filament.[12]
Hot gas between clusters in Shapley
Migkas and colleagues combined Suzaku and XMM-Newton observations to detect X-ray emission from a filament connecting two pairs of clusters in the Shapley supercluster.
They inferred a length of about 7.2 Mpc—roughly 23 million light-years—with gas near 10 million kelvin and only about ten free electrons per cubic meter.
Separating the filament’s signal from compact X-ray sources and neighboring clusters was essential. The study reveals one part of the diffuse gas reservoir and tests models of where that material resides.[13]
How does life in the web change a galaxy?
A galaxy moving through dense cluster plasma experiences pressure from the surrounding gas. This ram pressure can strip away part of its own gas, reducing the material available for future stars.
The loss of fresh supply can also gradually weaken star formation. These processes change the galaxy’s gas and activity; they do not automatically rearrange its existing stars into an elliptical shape.[8]
Stellar orbits and morphology respond to other processes, including the encounters discussed in Merging and Hierarchical Growth.
Galaxies also change the surrounding gas
Stars and active black holes return energy and material to their environments. Outflows spread heavy elements beyond the regions where they were made, and can move ordinary matter out of smaller halos into the wider web.[12]
The relationship runs both ways: the environment affects galaxies, while galaxies alter the temperature and chemistry of the gas around them.
What does the web tell us about cosmology?
The standard ΛCDM model combines cold dark matter with a cosmological constant, Λ, associated with accelerated expansion. It explains many measured properties of cosmic structure, while its components and detailed predictions remain subjects of testing.
The Cosmic Microwave Background constrains the early fluctuation statistics. Researchers evolve those initial conditions forward and ask whether the resulting galaxies, clusters, and mass distribution agree with later observations.[9]
Count clusters across time
The abundance of clusters of different masses depends on how rapidly structure grows and on the universe’s expansion history.
Researchers must establish which clusters a survey can detect and calibrate their masses. A bias in either step can resemble a change in cosmology.[17]
What do Ωm and σ₈ describe?
Ωm is the cosmic matter-density parameter. It includes both ordinary matter and dark matter; it does not describe dark matter alone.
σ8 measures the amplitude of present-day linear matter-density fluctuations after smoothing on the conventional radius of 8 h−1 Mpc. Here h expresses the Hubble constant in units of 100 kilometers per second per megaparsec.
These parameters are inferred together with other assumptions, rather than read directly from a single cluster image.[9]
What are researchers trying to resolve next?
Simulations reproduce the web by following the collective gravitational evolution of matter. Their numerical particles each represent large amounts of matter. Gas calculations add cooling, star formation, and feedback to investigate how galaxies and plasma occupy that structure.[16]
Increasing the modeled volume helps study rare clusters; improving resolution reveals smaller-scale detail. Comparing different calculations and observations helps separate physical effects from numerical limitations.
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Galaxy positions and distances
Wider redshift maps
DESI completed its original five-year survey observations in April 2026, with more than 47 million galaxies and quasars measured. Such samples sharpen tests of clustering and expansion.[14]
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Shapes and infrared observations
Euclid’s growing survey
ESA’s operating Euclid telescope combines visible imaging with near-infrared observations to study galaxy distributions and gravitational lensing across a broad sweep of cosmic history.[15]
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Matter between the galaxies
A more complete gas inventory
Radio bursts, X-ray spectra, and microwave measurements probe different properties of diffuse plasma. Together they test where the gas lies and how feedback has moved it.[10], [12], [13]
Open questions include how accurately cluster masses can be calibrated, how gas is divided between halos and filaments, and how strongly feedback changes the matter distribution on the scales measured by precision surveys.
Progress means explaining these observations together: the bright galaxies, the hot plasma, the lensing signal, and the comparatively quiet spaces between the dense structures.
The spaces between galaxies are part of the story.
Clusters concentrate matter, filaments connect regions, and voids reveal where the density is low. Their changing arrangement preserves evidence of early fluctuations and of the processes that have redistributed matter ever since.
Within this larger setting, some galactic centers become exceptionally luminous. The next article explores active galactic nuclei in the young universe and the growing black holes that power them.
Sources and further reading
Observations and mission descriptions reviewed in September 2026. Cluster composition, filament properties, and reconstructed maps depend on the systems and methods studied.
- Bond, Kofman & Pogosyan (1996) — How filaments of galaxies are woven into the cosmic webThe role of the gravitational tidal field in producing an interconnected web.
- Cautun et al. (2014) — Evolution of the cosmic webSimulated filaments, walls, nodes, and voids, and their changing mass and volume shares.
- Sheth & van de Weygaert (2004) — A hierarchy of voids: Much ado about nothingVoid expansion, merging, and the influence of the surrounding density field.
- Gonzalez et al. (2013) — Galaxy Cluster Baryon Fractions RevisitedThe inventory of hot gas, stars within galaxies, and diffuse intracluster stars.
- Lau, Kravtsov & Nagai (2009) — Residual Gas Motions in the Intracluster Medium and Bias in Hydrostatic Measurements of Mass Profiles of ClustersGas motions, pressure support, and the limitations of equilibrium-based cluster masses.
- Wallis et al. (2022) — Mapping dark matter on the celestial sphere with weak gravitational lensingReconstructing the integrated matter distribution from gravitational shear.
- Scoccimarro (2004) — Redshift-Space Distortions, Pairwise Velocities and NonlinearitiesHow galaxy motions affect the apparent structure of redshift maps.
- Tecce et al. (2010) — Ram pressure stripping in a galaxy formation model. I. A novel numerical approachGas removal as galaxies move through a dense surrounding medium.
- Planck Collaboration (2020) — Planck 2018 results. VI. Cosmological parametersCMB constraints on cosmological parameters and the statistics of density fluctuations.
- Sunyaev–Zeldovich Array / NASA LAMBDA — A Sunyaev–Zeldovich Effect PrimerThe spectral change produced when CMB photons scatter from energetic cluster electrons.
- Macquart et al. (2020) — A census of baryons in the Universe from localized fast radio burstsUsing radio-pulse dispersion to measure diffuse ionized ordinary matter.
- Connor et al. (2025) — A gas-rich cosmic web revealed by the partitioning of the missing baryonsA larger fast-radio-burst sample used to infer where the baryons reside.
- Migkas et al. (2025) — Detection of pure warm-hot intergalactic medium emission from a 7.2 Mpc long filament in the Shapley supercluster using X-ray spectroscopyAn individual filament studied with Suzaku and XMM-Newton.
- DESI Collaboration (2026) — DESI Reaches Mapping Milestone, Surpassing ExpectationsCompletion of the original five-year survey, with more than 47 million galaxies and quasars measured.
- European Space Agency — Euclid overviewAn operating survey mission combining visible imaging and near-infrared observations.
- Springel et al. (2005) — Simulations of the formation, evolution and clustering of galaxies and quasarsA numerical connection between dark matter assembly and large-scale galaxy populations.
- Kravtsov & Borgani (2012) — Formation of Galaxy ClustersCluster growth, hot plasma, and the use of cluster populations to test cosmology.
All articles in this chapter
- Gravitational Clumping and Density Fluctuations
- Population III Stars: The Universe’s First Generation
- Early Mini-Halos and Protogalaxies
- Supermassive Black Hole Seeds
- Primordial Supernovae: Element Synthesis
- Feedback Effects: Radiation and Winds
- Merging and Hierarchical Growth
- Galaxy Clusters and the Cosmic Web — you are here
- Active Galactic Nuclei in the Young Universe
- Observing the First Billion Years