According to the current cosmological model, there are five cosmic components
- Dark energy
- Dark matter
- Photons (most from CBR)
- Baryons (neutrons, protons, and electrons)
After the universe cooled off enough for plasma to engage with electrons to form elementary atoms (3000 °K and less),
baryons recombined to form basic atoms H and He. And this is what is thought as happened:
- H and He atoms were formed with a ratio H/He = 12:1.
- 90% of H and He atoms remained as diffuse ionized gas surounding galaxies.
- So, 83% of them were H atoms.
- 10% of baryons are found in stars, with 2% of them forming heavier elements.
- 8% of baryons remained as H and He atoms, with 7.4% being H atoms in stars.
It took about 1.3 million years for the 83% of original H atoms to start forming stars and galaxies.
It's assumed that there are 1086 H atoms in the observable universe, with 1023 stars (here is the link)
Estimations are that free Hydrogen has a density of 5/m3, what gives approximately 1059 free H atoms in the observable universe.
This is, approximately 1059 parts in a grand total of 1086 parts.
So, for this, the probability that a H atom has remained free for 13 billion years is close to one part in 1027 parts (almost zero).
What do you think about this? I've simplified a LOT of things and stuck with the observable universe only (as of today).