Final answer:
Helium existed in large quantities before star formation because the Big Bang model explains the production of significant amounts of helium in the universe's first few minutes, which cannot be matched by star-produced helium over the cosmos' 13.8 billion-year lifespan.
Step-by-step explanation:
The reason some, if not most of the helium must have existed before star formation is that there is an abundance of helium in the universe that cannot be accounted for solely by stellar nucleosynthesis over the 13.8 billion years of the universe's existence. To solve this mystery, we refer to the Big Bang model, which proposes that a significant amount of helium was created in the first few minutes after the Big Bang, during a period known as Big Bang nucleosynthesis. According to calculations, about 10 times more helium was produced during this period than has been generated by all the stars over the subsequent billions of years.
Observations support this model as well. The oldest stars and the most distant galaxies contain significant amounts of helium, indicating that this element was already present very early in the universe's history, corroborating the idea that helium synthesis took place during the Big Bang. In contrast, heavier elements like those we find on Earth needed the extreme conditions inside stars to form, and came into existence much later.