Primordial helium abundance
The primordial helium abundance
refers to the amount of helium in the universe produced shortly after the big bang
. Observationally, it comes from the observation that all stars, even very old ones, have a substantial amount of helium, and that this helium is much more than can be explained through stellar production.
The amount of primordial helium produced by big bang models varies based on the assumptions, but is consistently above 20%.