The cosmic web is the large-scale structure of the universe: a vast network of matter that connects galaxies across billions of light-years. Picture something like a three-dimensional spider web, with dense clumps of galaxies at the intersections, long bridges of matter stretching between them, and enormous empty bubbles filling the space in between. This structure isn’t random. It grew from tiny density fluctuations in the early universe, amplified by gravity over 13.8 billion years into the pattern we observe today.
The Building Blocks of the Web
The cosmic web has four distinct components: nodes, filaments, sheets, and voids. Nodes are the densest regions, where galaxy clusters containing hundreds or thousands of galaxies sit at the intersections of the web. Filaments are the long, thread-like bridges connecting these nodes, stretching anywhere from about 5 to 20 megaparsecs (roughly 16 to 65 million light-years). Sheets, sometimes called walls, are thinner, flatter structures that border the emptiest regions. And voids are the enormous near-empty bubbles that dominate the web’s volume.
The scale is staggering. Voids account for roughly 95 percent of the universe’s total volume. Most of the matter, however, is concentrated in the filaments, nodes, and sheets that weave around these empty spaces. A handful of lonely galaxies drift through voids, but the overwhelming majority line up along the web’s scaffolding.
What Filaments Are Made Of
Filaments are mostly dark matter, the invisible substance that makes up about 85 percent of all matter in the universe. Only 5 to 15 percent of a filament’s mass is ordinary (baryonic) matter: the atoms that make up gas, stars, and planets. Most of that ordinary matter isn’t in galaxies at all. It exists as a diffuse, superheated plasma known as the warm-hot intergalactic medium, with temperatures between 10 and 20 million degrees. That’s hot by any everyday standard, but it’s actually much cooler than the gas inside galaxy cluster cores, which can reach 100 million degrees.
This intergalactic plasma is where cosmologists believe much of the universe’s “missing” ordinary matter has been hiding. Calculations of how much baryonic matter the Big Bang produced don’t match the amount we can see in galaxies and galaxy clusters. Simulations show that the difference sits in these filaments, too hot to form stars but too diffuse to detect easily with telescopes.
How the Web Formed
The cosmic web traces its origins to the first fraction of a second after the Big Bang, when quantum fluctuations created tiny differences in density across the infant universe. Some patches were slightly denser than average, others slightly less so. As the universe expanded, gravity slowly amplified these differences. Denser regions pulled in surrounding matter, growing denser still, while underdense regions emptied out into voids.
The process wasn’t spherical. Matter didn’t simply collapse into round blobs. Instead, it followed a pattern first described mathematically by physicist Yakov Zel’dovich in the 1970s. His approximation showed that matter tends to collapse fastest along one axis, forming flat sheets first, then elongated filaments as a second axis collapses, and finally compact nodes where all three axes converge. This sequence, from sheets to filaments to nodes, is why the cosmic web looks like a web rather than a collection of isolated spheres. The Zel’dovich approximation remains one of the most important tools for understanding how the web’s skeleton emerged from the smooth, nearly uniform conditions of the early universe.
How Scientists Map Something This Large
You can’t photograph the cosmic web directly. Most of it is dark matter and diffuse gas, neither of which emits light that conventional telescopes can capture. Instead, astronomers use several indirect techniques.
One of the most powerful is the Lyman-alpha forest. When light from a distant quasar (an extremely bright galactic nucleus) travels toward Earth, it passes through clouds of neutral hydrogen gas strung along the cosmic web. Each cloud absorbs a narrow slice of the quasar’s light at a specific wavelength. By the time that light reaches a telescope, its spectrum is peppered with dozens or hundreds of absorption lines, creating a pattern that looks like a dense forest of dips. Each dip corresponds to a patch of gas at a particular distance, so astronomers can reconstruct the three-dimensional distribution of matter between here and the quasar. When quasars are close enough together on the sky, their overlapping sightlines allow full tomographic maps of the web’s density.
Another technique is weak gravitational lensing. Dark matter warps the fabric of space, bending light from distant background galaxies. The distortions are subtle, slightly stretching galaxy shapes in a coherent pattern. By measuring these tiny shape distortions across millions of galaxies, astronomers can reverse-engineer where the dark matter sits, effectively mapping the invisible scaffold of the web.
Simulating the Web on Supercomputers
Because the cosmic web spans billions of light-years and evolved over billions of years, computer simulations are essential for understanding it. The IllustrisTNG project is one of the most detailed to date. It models cubic volumes of the universe up to 300 megaparsecs (about a billion light-years) per side, tracking matter from a redshift of 20 (when the universe was only a few hundred million years old) all the way to the present. These simulations include gravity, gas dynamics, magnetic fields, black hole formation and feedback, and stellar evolution, producing virtual universes that closely match what telescopes actually observe.
The results consistently confirm the same picture: dark matter collapses first into a web-like framework, and ordinary matter follows, falling into the gravitational valleys that dark matter carved. Galaxies form at the densest points along filaments and especially at nodes. The simulations also reveal how environment shapes galaxies. Those living in dense filaments tend to be redder, older, and more massive, while those drifting in voids tend to be bluer and still forming stars.
What DESI Is Revealing Now
The Dark Energy Spectroscopic Instrument, or DESI, has created the largest three-dimensional map of the universe ever made, reaching back over 11 billion years of cosmic time. By measuring the spectra of millions of galaxies and quasars, DESI tracks a feature called baryon acoustic oscillations: a fixed-size ripple imprinted in the distribution of matter by sound waves in the early universe. Because this ripple has a known physical size, it works as a “standard ruler,” letting scientists measure how fast the universe was expanding at different points in its history.
DESI has now measured this signal at seven different epochs, spanning from 3 billion to 11 billion years ago, with better than 1 percent precision. The results broadly agree with the standard cosmological model, which includes dark matter and dark energy as driving forces. But there are hints that dark energy may not be constant over time. If confirmed, that would reshape our understanding of what’s pushing the universe’s expansion to accelerate, and by extension, how the cosmic web will continue to evolve.
Why the Cosmic Web Matters
The cosmic web isn’t just a curiosity. It’s the context in which every galaxy, including the Milky Way, exists. The web determines where galaxies form, how they grow, and how they interact. Galaxies falling along a filament toward a cluster get stripped of gas and stop forming stars. Galaxies in voids evolve in relative isolation. The web also serves as a testing ground for fundamental physics: its structure encodes information about dark matter, dark energy, and the initial conditions of the universe. Every improvement in mapping the web gives physicists a sharper tool for understanding what the universe is made of and why it looks the way it does.