https://blogs.scientificamerican.com/life-unbounded/a-molecular-universe/

It’s a relatively little-known fact outside of astrophysics that the key to the first stars in the universe, and the earliest structures condensing out of the primordial murk, was chemistry.

Specifically, the key was the formation of molecular hydrogen or H2. A pair of atoms bonded together and capable of rotating and vibrating. A few years ago I wrote about some of the details in this column. In brief, without the formation of molecular hydrogen it’s very hard for cosmic gas to cool off. If it can’t cool off it can’t condense to make stars, and for those stars to forge the first heavier elements that act as much more efficient coolants for subsequent generations of forming objects. (It could be argued that helium hydride, forming before molecular hydrogen, was also key, but more so for enabling the formation of molecular hydrogen in the first place).