How galaxies cluster into massive structures shaped by dark matter and primordial fluctuations
More Than Individual Galaxies
Our Milky Way is just one of billions of galaxies. However, galaxies do not float randomly: they cluster into superclusters, filaments, and sheets, separated by vast voids almost devoid of luminous matter. All these large-scale structures form a network stretching across hundreds of millions of light-years, often called the “cosmic web.” This complex network forms primarily due to the dark matter scaffold, whose gravitational pull organizes both dark and baryonic matter into cosmic “roads” and voids.
Dark matter distribution, shaped by primordial fluctuations in the early Universe (amplified by cosmic expansion and gravitational instability), creates the seeds of galaxy halos. Galaxies later form within these halos. Observing these structures and comparing them with theoretical simulations has become a cornerstone of modern cosmology, confirming the ΛCDM model on the largest scales. Below is an overview of how these structures were discovered, how they evolve, and current research horizons aiming to better understand the cosmic web.
2. Historical Development and Survey Overviews
2.1 Early Signs of Clustering
The first galaxy catalogs (for example, Shapley's observations of rich clusters in the 1930s, later redshift surveys like the CfA Survey in the 1980s) showed that galaxies indeed cluster into large structures much bigger than individual clusters or groups. Superclusters, such as the Coma Supercluster, suggested that the nearby Universe has a filamentary distribution.
2.2 Redshift Surveys: Pioneers 2dF and SDSS
2dF Galaxy Redshift Survey (2dFGRS) and later the Sloan Digital Sky Survey (SDSS) significantly expanded galaxy maps to hundreds of thousands, and later to millions of objects. Their three-dimensional maps clearly revealed the cosmic web: long filaments of galaxies, huge voids where galaxies are almost absent, and massive superclusters forming at intersections. The largest filaments can stretch for hundreds of megaparsecs.
2.3 Modern Era: DESI, Euclid, Roman
Current and upcoming surveys, such as DESI (Dark Energy Spectroscopic Instrument), Euclid (ESA), and the Nancy Grace Roman Space Telescope (NASA), will further deepen and expand these redshift maps to tens of millions of galaxies with greater redshifts. They aim to study the evolution of the cosmic web from early epochs and more precisely assess the interplay of dark matter, dark energy, and structure formation.
3. Theoretical Foundations: Gravitational Instability and Dark Matter
3.1 Primordial Fluctuations from Inflation
In the early Universe, during inflation, quantum fluctuations turned into classical density perturbations spanning various scales. After inflation ended, these perturbations became the seeds of cosmic structures. Since dark matter is cold (becoming non-relativistic early), it began to cluster fairly quickly once it decoupled from the hot radiation background.
3.2 From Linear Growth to Nonlinear Structure
As the Universe expanded, regions with slightly higher than average density gravitationally attracted more matter, and the density contrast grew. Initially, this process was linear, but in some areas it became nonlinear until those regions collapsed into gravitational halos. Meanwhile, lower density regions expanded faster, forming cosmic voids. The cosmic web arises from this mutual gravitational interaction: dark matter forms the framework into which baryons fall, forming galaxies.
3.3 N-body Simulations
Modern N-body simulations (Millennium, Illustris, EAGLE, and others) track billions of particles representing dark matter. They confirm the web-like distribution – filaments, nodes (clusters), and voids – and show how galaxies form in dense halos at these node intersections or along filaments. These simulations use initial conditions from the CMB (Cosmic Microwave Background) power spectrum, demonstrating how small amplitude fluctuations grow into the structures observed today.
4. Cosmic Web Structure: Filaments, Voids, and Superclusters
4.1 Filaments
Filaments are connections between massive clusters of "nodes." They can stretch tens or even hundreds of megaparsecs, containing various galaxy clusters, groups, and intergalactic gas. Some observations show faint X-ray (X) or hydrogen HI radiation connecting the clusters, indicating the presence of gas. These filaments act like highways along which matter moves from less dense regions toward denser nodes due to gravity.
4.2 Voids
Voids are vast, low-density regions with very few galaxies. They typically span about 10–50 Mpc in diameter but can be larger. Galaxies inside voids (if any) are often highly isolated. Voids expand somewhat faster than denser regions, potentially influencing galaxy evolution. It is estimated that ~80–90% of cosmic space consists of voids, containing only about ~10% of all galaxies. The shape and distribution of these voids allow testing hypotheses about dark energy or alternative gravity models.
4.3 Superclusters
Superclusters are usually not fully gravitationally bound but form large-scale overdensities encompassing multiple clusters and filaments. For example, the Shapley supercluster or the Hercules supercluster are among the largest known structures of this type. They define the large-scale environment for galaxy clusters but may not become a single gravitationally bound entity over cosmic timescales. Our Local Group belongs to the Virgo supercluster, also called Laniakea – here hundreds of galaxies are concentrated, with the Virgo cluster at its center.
5. The Role of Dark Matter in the Cosmic Web
5.1 Cosmic Framework
Dark matter, being collisionless and constituting the majority of matter, forms halos at nodes and along filaments. Baryons, which interact electromagnetically, later condense into galaxies within these dark matter halos. Without dark matter, baryons alone would struggle to form massive gravitational wells early enough to produce the structures observed today. N-body simulations removing dark matter show a completely different distribution, inconsistent with reality.
5.2 Observational Confirmation
Weak gravitational lensing (cosmic shear) over large sky areas directly measures the mass distribution, which coincides with filamentary structures. X-ray and Sunyaev–Zeldovich (SZ) effect observations in clusters reveal hot gas accumulations that often correspond to dark matter gravitational potentials. The combination of lensing, X-ray data, and galaxy cluster distribution strongly supports the importance of dark matter in the cosmic web.
6. Impact on Galaxy and Cluster Formation
6.1 Hierarchical Merging
Structures form hierarchically: smaller halos merge into larger ones over cosmic time. Filaments provide a continuous flow of gas and dark matter into cluster nodes, further growing them. Simulations show that galaxies located in filaments experience faster material inflow, which affects their star formation history and morphological transformations.
6.2 Environmental Impact on Galaxies
Galaxies in dense filaments or cluster centers experience ram-pressure stripping, potential tidal disruptions, or gas depletion issues, which can lead to morphological changes (e.g., spirals transforming into lenticular galaxies). Meanwhile, galaxies in voids may remain gas-rich and form stars more actively, as they have fewer interactions with neighbors. Thus, the cosmic web environment significantly influences galaxy evolution.
7. Future Surveys: Detailed Web Map
7.1 DESI, Euclid, Roman Projects
DESI (Dark Energy Spectroscopic Instrument) collects redshifts of ~35 million galaxies/quasars, enabling 3D cosmic web maps up to about z ~ 1–2. Simultaneously, Euclid (ESA) and the Roman Space Telescope (NASA) will provide extremely wide-field images and spectroscopic data of billions of galaxies, allowing measurements of lensing, BAO, and structure growth to refine dark energy and cosmic geometry. These next-generation surveys will allow unprecedentedly precise "weaving" of the web map up to ~z = 2, covering an even larger portion of the Universe.
7.2 Spectral Line Maps
HI intensity maps (intensity mapping) or CO line maps can enable faster observations of large-scale structure in redshift space without resolving every individual galaxy. This method accelerates surveys and provides direct information on matter distribution over cosmic times, offering new constraints on dark matter and dark energy.
7.3 Cross-Correlations and Multi-Messenger Methods
Combining data from different cosmic probes – CMB lensing, weak lensing of galaxies, X-ray cluster catalogs, 21 cm intensity maps – will allow precise reconstruction of the three-dimensional density field, filaments, and matter flow fields. Such a combination of methods helps test gravity laws on large scales and compare ΛCDM predictions with possible modified gravity models.
8. Theoretical Studies and Unanswered Questions
8.1 Small-Scale Discrepancies
Although the cosmic web largely matches ΛCDM well, discrepancies are observed in certain small-scale regions:
- Cusp–core problem in the rotation curves of dwarf galaxies.
- Missing satellites problem: fewer dwarf halos are found around the Milky Way than expected from simple simulations.
- Plane of satellites phenomenon or other distribution discrepancies in some local galaxy groups.
This may imply that important baryonic feedback processes or new physics (e.g., warm dark matter or interacting dark matter) are needed, which alter structure on scales smaller than Mpc.
8.2 Early Universe Physics
The primordial fluctuation spectrum observed in the cosmic web is related to inflation. Studies of the web at higher redshifts (z > 2–3) could reveal subtle signs of non-Gaussian fluctuations or alternative inflation scenarios. Meanwhile, filaments and baryon distribution during the reionization epoch represent another observational "horizon" (e.g., via 21 cm tomography or deep galaxy surveys).
8.3 Testing Gravity on Large Scales
Theoretically, by studying how filaments form over cosmic time, one can test whether gravity conforms to General Relativity (GR) or exhibits deviations under certain conditions in large-scale superclusters. Current data support the standard growth of gravity, but a more detailed map in the future may reveal subtle deviations important for f(R) or "braneworld" theories.
9. Conclusion
The cosmic web – the great weave of filaments, voids, and superclusters – reveals how the Universe's structure unfolds from the growth of gravitational primordial density fluctuations governed by dark matter. Its discovery through large redshift surveys and comparison with reliable N-body simulations makes it clear that dark matter is an essential "scaffold" for the formation of galaxies and clusters.
The cosmic web is arranged along these filaments, flowing into cluster nodes, while large voids remain some of the emptiest regions of space. This arrangement, extending over hundreds of megaparsecs, reveals the hierarchical growth features of the Universe, perfectly consistent with ΛCDM and confirmed by CMB anisotropies and the entire chain of cosmic observations. Reviews of current and future projects will allow an even more detailed "grasp" of the three-dimensional cosmic web image, better understanding the evolution of the Universe's structure, the nature of dark matter, and testing whether standard laws of gravity hold on the largest scales. This cosmic web is a grand, interconnected motif and the very "fingerprint" of cosmic creation from the earliest moments to the present day.
Literature and Further Reading
- Gregory, S. A., & Thompson, L. A. (1978). “Superclusters of galaxies.” The Astrophysical Journal, 222, 784–796.
- de Lapparent, V., Geller, M. J., & Huchra, J. P. (1986). "A slice of the universe." The Astrophysical Journal Letters, 302, L1–L5.
- Colless, M., et al. (2001). “The 2dF Galaxy Redshift Survey: spectra and redshifts.” Monthly Notices of the Royal Astronomical Society, 328, 1039–1063.
- Tegmark, M., et al. (2004). “Cosmological parameters from SDSS and WMAP.” Physical Review D, 69, 103501.
- Springel, V., et al. (2005). "Simulations of the formation, evolution and clustering of galaxies and quasars." Nature, 435, 629–636.