What is a Cosmic Web?
On the largest scales there is a pattern. Galaxies are not sprinkled evenly through space — they lie along filaments and sheets that meet at dense knots, wrapped around voids hundreds of millions of light years across that are very nearly empty. This is the cosmic web, and it is the largest structure that exists.
It grew from almost nothing. The early universe was smooth to about one part in a hundred thousand, and the cosmic microwave background is a photograph of exactly how smooth. But gravity is unstable in the direction of clumping: regions a fraction denser than average pull in more matter, which makes them denser still. Over 13.8 billion years those imperceptible ripples became everything. Collapse does not happen evenly in all three dimensions either — a region collapses first along one axis into a sheet, then along a second into a filament, then to a knot — which is why the web has the shape it has rather than being a scatter of blobs.
Most of what draws the pattern cannot be seen. Dark matter outweighs ordinary matter roughly five to one and did the gravitational work; galaxies formed where its filaments were densest, so the glowing part of the web is a sparse tracer decorating a scaffold that emits nothing. Strung between the galaxies there is also gas — the warm-hot intergalactic medium — that holds a large fraction of all the ordinary atoms in the universe and was extremely difficult to detect precisely because it is so thin.
Mapping it took decades of patient work. Every galaxy's distance has to be measured individually from its redshift, and surveys like the Sloan Digital Sky Survey have now done that for millions of them, turning a flat picture of the sky into a three-dimensional map. What those maps show matches what simulations of a universe made mostly of cold dark matter predict, in detail — which is one of the strongest pieces of evidence that the standard model of cosmology is right.