cosmic web
Zoom far enough out from any single galaxy and a stunning pattern appears. Galaxies are not scattered randomly through space, nor spread evenly. They string together along glowing filaments, pile up where filaments cross into dense clusters, and leave behind vast, nearly empty bubbles called voids. The whole arrangement looks like a tangled spiderweb, or the froth of soap bubbles, threaded across hundreds of millions of light-years. This is the cosmic web — the largest pattern in nature.
The web has a hierarchy of parts. Voids are enormous round emptinesses, tens to hundreds of millions of light-years across, where almost no galaxies live. Walls (or sheets) are flat surfaces of galaxies wrapping the voids. Filaments are the thread-like bridges where matter funnels along, and at the junctions where several filaments meet sit galaxy clusters, the densest knots. Crucially, this scaffolding is sculpted by dark matter: invisible dark matter clumped first, under gravity, into a web of filaments and halos, and ordinary gas then flowed downhill into those dark wells to form galaxies. We see the galaxies lighting up the web, but the dark matter is the frame.
The cosmic web is one of dark matter's clearest fingerprints. Computer simulations starting from tiny early-universe density ripples, with cold dark matter and dark energy, naturally grow a web that matches the real one mapped by enormous galaxy redshift surveys. The size and shape of the voids and filaments encode the recipe of the universe — how much matter, how much dark energy — so the web is not just a pretty picture but a precision cosmological measuring tool.
Galaxy redshift surveys like the Sloan Digital Sky Survey mapped the positions of millions of galaxies in 3-D. The maps reveal long filaments such as the 'Sloan Great Wall,' a sheet of galaxies over a billion light-years long, separating voids hundreds of millions of light-years across — the cosmic web laid bare.
Redshift surveys reveal galaxies strung along filaments around vast empty voids — the cosmic web.
The galaxies are only the lit-up parts of the web; most of the mass tracing the filaments is dark matter, plus diffuse intergalactic gas. The web is also not a fixed structure — it keeps growing and sharpening as the universe expands.