Large-scale structure — the cosmic web

The hierarchy nests. Stars gather into galaxies; galaxies fall together into groups (like our Local Group) and rich clusters (like the Virgo Cluster, over a thousand galaxies bound in one gravitational well). Groups and clusters are themselves drawn into superclusters — loose, sprawling concentrations tens of megaparsecs across. Our Local Group sits on the outskirts of the Virgo Supercluster, ~33 megaparsecs (about 110 million light-years) wide and centred on the Virgo Cluster.

Superclusters are not the top. Map the motions of galaxies rather than just their positions and the Virgo Supercluster turns out to be one lobe of a far larger basin of attraction — Laniakea, some 160 megaparsecs across, defined in 2014 as the region where galaxy flows all stream toward a common gravitational focus near the Great Attractor. Laniakea in turn borders neighbouring superclusters across the sky.

Between the bright filaments lie the voids — enormous, roughly spherical regions tens of megaparsecs across that are nearly empty of galaxies. Most of the volume of the universe is void; most of the galaxies live in the thin walls and threads between them. Seen at the largest scale, the distribution looks like a sponge, or a web of glowing threads around dark bubbles.

This pattern is not decorative — it is the fossil of the early universe. Tiny density ripples present when the universe was 380,000 years old, imprinted on the cosmic microwave background, were amplified over billions of years by gravity: dense regions pulled in more matter and grew, empty regions emptied further. Dark matter, which outweighs ordinary matter roughly five to one, did most of the pulling and laid down the scaffolding the visible galaxies later lit up.

The cosmic web is also the frontier of the honest map. At these distances positions come from redshift, not a tape measure, and the finest structure is model-dependent — which is why Orrery renders the outer scale shells as explicit schematics, real distances on an illustrative layout, rather than pretending to a precision no survey yet has.