the first few minutesReputable sourceWell documented
The First Elements: Big Bang Nucleosynthesis
On the timeline · around the first few minutes · The First Instants
What happened
In the first few minutes, protons and neutrons fused to build the lightest atomic nuclei — hydrogen, helium and a trace of lithium. After about five minutes the universe had expanded and cooled too much for any more to form.
Why it matters
This set the basic chemistry of the cosmos. Nearly all the hydrogen and helium in existence today was created in these opening minutes.
Sources
- European Space Agency. So, how did everything start? · Reputable source