The main source of elements heavier than helium is nucleosynthesis driven by supernova explosions. Hydrogen, deuterium, and helium are believed to have appeared about 20 minutes after the Big Bang in the proportions 3 protons to 10-4 deuterons to one alpha particle. Nucleosynthesis then ceased because the temperature fell below the level at which further nuclear fusion can occur.
The first stars, consisting of these primordial elements, appeared about 100 million years after the Big Bang. Most of these early stars were huge, and would end their very short lives as pair-creation instability supernovae. Elements as heavy as iron were formed during the life and death of this earliest generation of stars. Enormous supernovae and a small universe suggest that virtually all interstellar and intergalactic gas increased rapidly in metallicity after another 100 million years. Indeed, observations of galaxies from 500 million years after the Big Bang reveal small but observable metallicities, not atypical of current blue galaxies.
Recently, a report appeared describing the discovery at the Keck observatory of primordial gas clouds having hydrogen/helium and hydrogen/deuterium ratios and apparently zero metallicity. This gas is consistent in every way with the primordial gas resulting from Big Bang nucleosynthesis. The primordial gas clouds lay at a distance corresponding to two billion years after the Big Bang. This observation was made using a novel technique to measuring the absorption of gas directly between us and a very early quasar.
These observations lead to a mystery. How can clouds of pristine Big Bang gases exist after two billion years of ongoing nucleosynthesis of metals in the universe? The early universe may have been split up into well-isolated domains, perhaps by dark matter, perhaps by gravitational domain walls, perhaps by less well known effects. What is clear is that we keep being surprised by the universe.
For more information on the image used in this story, check out the Hubble site