The cosmic microwave background, discovered by Penzias and Wilson (1965), provides the strongest evidence for the hot origin of the Universe - otherwise known as the hot Big Bang. In this grand model, the radiation and baryonic matter were in thermodynamic equilibrium at sufficiently early times, which involved the constituent fundamental particles interacting with photons via Compton scattering. As the Universe expanded and photons redshifted so the temperature of the background radiation cooled. The radiation field last interacted with matter at a redshift z 1000, or some 106 yrs after the Big Bang - at which time the temperature of the radiation had cooled to 3000° K, enabling electrons to be captured forming neutral atoms.
It is believed that structures such as galaxies started out as small density fluctuations in this primordial soup of matter and radiation, growing by gravitational instability into larger overdensities as gravitationally bound systems were formed. The dense clumps of material in the gravitational potential wells caused collisional heating of the baryonic material, allowing rapid cooling of the gas by line radiation. Subsequently, larger clumps were formed as sub-clumps of material merged and combined - with the final state of the bound material governed by angular momentum conservation. The first stars formed in the dense cores, which enriched the primordial gas with heavy elements as a result of supernovae explosions.
In recent years there has been substantial empirical underpinning of this general picture of the early growth of structure. For example, the earliest density perturbations have now been detected as small temperature fluctuations (T/T 10-6) in the cosmic microwave background by the Cosmic Microwave Background Explorer (COBE) satellite (see Smoot and Keay 1993). In addition, semi-analytical models of the growth of structure, which incorporate most of the known physical processes, have had success in reproducing many of the optical properties of galaxies in the most distant surveys (e.g. Baugh et al. 1998).