What hypothetical observation would challenge our current understanding of the Big Bang theory regarding helium abundance?

The hypothetical observation that would challenge our current understanding of the Big Bang theory regarding helium abundance is the discovery of a galaxy with a helium abundance of only 10% by mass. The Big Bang nucleosynthesis model predicts that the universe's primordial nucleosynthesis should produce helium in substantial quantities, specifically around 25% by mass, alongside hydrogen.

If a galaxy were observed with only 10% helium by mass, this would suggest a significant deviation from the established predictions of the Big Bang theory. Such a finding would imply that either the processes that led to the formation of galaxies in that region differed greatly from those outlined in the Big Bang model, or that there were unaccounted-for mechanisms affecting helium production and retention in that galaxy.

The other scenarios presented don't directly contradict established helium abundance predictions from the Big Bang. For instance, a galaxy composed entirely of hydrogen would not necessarily invalidate the Big Bang theory; instead, it might suggest certain evolutionary processes that favor hydrogen retention or lack of stellar formation. Similarly, a galaxy with no hydrogen content might be more indicative of unique astrophysical conditions or evolutionary history rather than disproving fundamental aspects of the Big Bang theory. Detecting a supernova event with a high helium signature could elevate discussions about stellar processes