The years 1965-1975 have witnessed the establishment of the primordial nucleosynthesis, (also called BBN for Big-Bang nucleosynthesis). The problem of the origin of the light elements from 2D to 11B was gradually solved, and it became clear that the four isotopes 2H, 3He, 4He, and 7Li, were bona-fide relics from these ancient hot times. It is worthwhile at this point to say that the solar wind experiment of Johannes Geiss and his Berne collaborators (Bühler et al 1971), in giving reliable non-terrestrial values for the helium isotopic ratio, played an important role in the set-up of this achievement.
As mentioned before, this calculation also provides an estimate of the baryonic density of the universe. Allowing for the uncertainty in the value of H, the Hubble constant, the baryonic density is between one and twelve percent of the critical density. The best estimates of the total (baryonic and non-baryonic) cosmic density, from dynamic effects on galactic motions, yields values around ten per cent of the closure density. Thus there is no disagreement between the (baryonic) density given by BBN and the dynamic evaluation. There is no sound proof of the existence of a non-baryonic matter contributing in a major way to the total density of the universe.
In the last decade, the success of primordial nucleosynthesis has become a major card in cosmological studies and explorations. It has been used as ground basis for testing new hypotheses or even conceptual framework. It has served as an anchor for theories - and theorists - to keep contact with observations, (a fundamental but sometimes overlooked condition for successful scientific progresses). There is already a large cemetery of cosmological models, brillant or not, which have died because they could not reproduce in a convincing matter the success of the simple BBN. I want to discuss some areas of research in which primordial nucleosynthesis has played a preeminent role.