A ‘flat’ universe with Ω = 1, attractive on theoretical grounds, can only be reconciled with virial-type analyses if galaxies are more ‘clumped’ than the overall mass distribution even on scales up to 10–20 Mpc. Formation of conspicuous galaxies must be favored in high-density regions, and inhibited in voids. Mechanisms are discussed for bringing this about, particularly in the context of the ‘cold dark matter’ cosmogony. There are two forms in which energy from early galaxies could provide a pressure that prevents condensation of further galaxies ≳20 Mpc away: (i) fast particles, or (ii) UV radiation pressure, coupled to the intergalactic gas via Lyman α scattering. Other processes for ‘biasing’ bright galaxies towards high-density regions are outlined. If the universe has Ω = 1, with baryons contributing Ωb = 0.1, then up to 90 per cent of these baryons could remain uncondensed in intergalactic space or be in systems with M/L > 100 solar units.