The universe's matter clusters hierarchically into filaments, sheets, and voids. Galaxy clusters form at filament intersections; vast voids contain few galaxies. This cosmic web structure emerges from gravitational instability amplifying tiny initial density fluctuations. Surveys reveal the structure and constrain the matter content and expansion history of the universe.
From your study of dark matter and dark energy, you know that most of the universe's mass-energy is invisible and that cosmic expansion is accelerating. From the Hubble law, you know that the universe is expanding and that distance correlates with recession velocity. The large-scale structure of the universe is the story of how gravity, working with and against this expansion, sculpted matter into the patterns we observe today — a story written in the three-dimensional positions of billions of galaxies.
If you could zoom out far enough to see the universe on scales of hundreds of millions of light-years, galaxies would not appear uniformly scattered. Instead, they trace out a vast network called the cosmic web: long, thin filaments of galaxies and gas connecting dense clusters at their intersections, with thin sheets or walls bounding enormous, nearly empty voids that can span 100 million light-years or more. The densest concentrations — galaxy clusters containing thousands of galaxies — sit at the nodes where multiple filaments meet. This web-like pattern is one of the most striking features of the observed universe, revealed by galaxy redshift surveys like the Sloan Digital Sky Survey (SDSS) and the 2dF Galaxy Redshift Survey, which mapped the three-dimensional positions of millions of galaxies.
The cosmic web is the end product of gravitational instability acting over 13.8 billion years. In the very early universe, matter was distributed almost — but not perfectly — uniformly. Tiny density fluctuations, with amplitudes of roughly one part in 100,000 (visible as temperature variations in the cosmic microwave background), provided the seeds. Regions slightly denser than average had slightly stronger gravitational pull, attracting more matter from their surroundings and growing denser still. Regions slightly less dense lost matter to their neighbors and became emptier. Over cosmic time, this positive feedback — denser regions pulling in more material, under-dense regions evacuating — produced the dramatic contrast we see today: filaments and clusters separated by vast voids.
Dark matter plays the dominant role in this process. Because dark matter does not interact with light or experience radiation pressure, it began clumping gravitationally earlier than ordinary (baryonic) matter, which was still coupled to radiation in the early universe. Dark matter formed the gravitational scaffolding — the skeleton of the cosmic web — and baryonic matter subsequently fell into these dark matter structures, forming the visible galaxies we observe tracing the web. Computer simulations of cosmic structure formation, such as the Millennium Simulation and IllustrisTNG, model this process by evolving billions of dark matter particles under gravity from initial conditions matching the CMB fluctuations. These simulations reproduce the observed cosmic web with remarkable fidelity, providing strong evidence that our understanding of gravitational structure formation — seeded by quantum fluctuations, shaped by dark matter, and slowed by dark energy's accelerating expansion — is fundamentally correct. The statistical properties of the cosmic web — particularly the two-point correlation function and the baryon acoustic oscillation (BAO) signal — serve as precision tools for measuring the universe's matter content, expansion rate, and geometry.