Start off by populating the universe with loose atoms.
You're going to have quite a lot of H, some He, and ever decreasing quantities of larger atoms. The abundance of atoms in our galaxy by mass is 74% H, 24% He, 1% O, 0.5% C, 0.1% Ne & Fe & N, 0.05% Si & Mg & S. When you break it down by quantity of atoms, the spread only grows wider. 92% H, 7.5% He, 0.1% O, 0.05% C, 0.01% N & Ne, and ~0.003% Mg & Si & Fe & S.
Now, for each atom, you're going to work the room at the universal party. If you're an H, you show up early to the party, and you're very likely to run into another H and really hit it off. So most of the H in the universe is going to pair up as H2 before anyone else even shows up. He and Ne don't really want to be at the party. They hang out in the corners and stare at their own quarks. So let's just pair up all those H atoms into H2 and ignore the He and Ne.
We're now at 99.6% H2, 0.2% O, 0.1% C, 0.02% N, ~0.005% Mg & Si & Fe & S among atoms that actually want to bond.
So C, N, Mg, Si, and Fe are pretty chill. If they happen to meet up with a lone H, they get along together pretty well. Otherwise, they're content to circulate around the party and meet other atoms looking for a good bond.
But O and S are a little crazy. They are not going home alone, ever. And usually, they're going to hook up with the first other atom (or molecule) they meet at the party. Almost every O that joins the party is going to end up as the meat in an H sandwich and leave the party. By coincidence, some will become O2 and be content. A bit less will become CO, or NO, and a very tiny amount will end up in MgO, SiO, FeO, or SO. And those will stay at the party, looking to bond more.
So just by random meetings based on relative abundance, it is clear that H2O is going to be third among discrete chemical units, and second among bonded molecules. If O were less abundant, or less ravenous for electrons, it might be different, but it's pretty safe to say that O will try to react with anything it meets, and it's very likely to meet an H2.
Thanks for a detailed reply. My comment wasn't trying to be snarky, I really would like to know a reputable source for this statement.
As far as I understand it's much easier for us to know distribution of different elements that it is to know distribution of different molecules in the universe. I assumed that it wouldn't be that popular given that it does not take part in a nuclear fusion.
You're going to have quite a lot of H, some He, and ever decreasing quantities of larger atoms. The abundance of atoms in our galaxy by mass is 74% H, 24% He, 1% O, 0.5% C, 0.1% Ne & Fe & N, 0.05% Si & Mg & S. When you break it down by quantity of atoms, the spread only grows wider. 92% H, 7.5% He, 0.1% O, 0.05% C, 0.01% N & Ne, and ~0.003% Mg & Si & Fe & S.
Now, for each atom, you're going to work the room at the universal party. If you're an H, you show up early to the party, and you're very likely to run into another H and really hit it off. So most of the H in the universe is going to pair up as H2 before anyone else even shows up. He and Ne don't really want to be at the party. They hang out in the corners and stare at their own quarks. So let's just pair up all those H atoms into H2 and ignore the He and Ne.
We're now at 99.6% H2, 0.2% O, 0.1% C, 0.02% N, ~0.005% Mg & Si & Fe & S among atoms that actually want to bond.
So C, N, Mg, Si, and Fe are pretty chill. If they happen to meet up with a lone H, they get along together pretty well. Otherwise, they're content to circulate around the party and meet other atoms looking for a good bond.
But O and S are a little crazy. They are not going home alone, ever. And usually, they're going to hook up with the first other atom (or molecule) they meet at the party. Almost every O that joins the party is going to end up as the meat in an H sandwich and leave the party. By coincidence, some will become O2 and be content. A bit less will become CO, or NO, and a very tiny amount will end up in MgO, SiO, FeO, or SO. And those will stay at the party, looking to bond more.
So just by random meetings based on relative abundance, it is clear that H2O is going to be third among discrete chemical units, and second among bonded molecules. If O were less abundant, or less ravenous for electrons, it might be different, but it's pretty safe to say that O will try to react with anything it meets, and it's very likely to meet an H2.