Quasars, the expanding Universe and large-scale structure

Galaxies are not scattered evenly: they form groups (the Local Group, ≈ 80 galaxies) and clusters (the Virgo Cluster at ≈ 16.5 Mpc, ≈ 1300–2000 galaxies). Clusters join into superclusters: the Local Group belongs to the Virgo Supercluster, which lies within the still larger Laniakea Supercluster (≈ 160 Mpc across, ≈ 10¹⁷ M☉). On the largest scales galaxies lie along filaments and walls, between which are almost empty voids tens to hundreds of Mpc across – the cosmic web (the Great Wall, 1989). On scales larger than ≈ 100–300 Mpc the Universe is homogeneous and isotropic – the cosmological principle. Dark matter’s gravity was decisive in forming the web, whose seeds were tiny irregularities of ≈ 10⁻⁵ in the cosmic microwave background. 3D maps are made from galaxy redshifts (SDSS, DESI).