The hydrogen was there before the stars formed: it is one of the left overs from the Big Bang. Some hydrogen is recycled from stellar explosions and stellar winds though. They are the only source of elements heavier than Helium (or Lithium if you are strict about the only part). Stars then formed in regions where clouds of hydrogen contracted under there own gravity. So indeed the question is more how hydrogen managed to cluster dense enough that stars and galaxies could form from the primordial hydrogen than how the hydrogen gathered near the stars. This is actually a still unsolved problem for the first stars and galaxies: most scenario's need heavier elements to efficiently radiate away energy, but stars had to have formed for the heavier elements to form: a nice chicken and egg problem.
Not sure the word "huge" is needed but the basic question of interest is whether or not most of the baryons in the Universe are inside or outside of gravitational potentials (e.g. galaxies).
Agreed that understanding how the first generation of stars formed inside galaxy halos remains largely unknown.
@G Bothun,
I think I read that the amount of Hydrogen in intergalactic clouds (i.e. not gravitationally bound), exceeds the amount of hydrogen in galaxies by an order of 10 or so.
See http://arxiv.org/abs/1001.3692
http://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium and http://en.wikipedia.org/wiki/Intergalactic_medium#Intergalactic_space - briefly,
"... Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm-hot, rarefied state [the warm-hot intergalactic medium].[108][110][111]"
Please see the Wikipedia sources above for these and other reference links.
Also, http://en.wikipedia.org/wiki/Intracluster_medium#Composition states
"... Most of the baryons in the cluster (80-95%) reside in the ICM, rather than in the luminous matter, such as galaxies and stars."
However, no references are cited...
It is only very massive clusters of galaxies with deep central potentials that produce very hot gas (X-ray emission) in which the majority of the baryons are in the hot gas.
A more typical cluster does not have such a high fraction of baryonic mass in
in the form of hot gas.
And again, the WIM/Cosmic Web has only been partially detected - data are constant with "1/2" but do not independently establish that level
The abstract of http://arxiv.org/abs/1001.3692 states:
"... we estimate a baryon over-density of ~ 30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations."
To clarify...
Rogier had said "... Hydrogen in intergalactic clouds (i.e. not gravitationally bound), exceeds the amount of hydrogen in galaxies by an order of 10 or so"
I think he may have been recalling that the vast majority of hydrogen in galaxy clusters is thought to be contained within the intracluster medium, rather than within galaxies.
Hydrogen in intergalactic clouds is thought to "... contain 40–50%[1] of the baryons (that is, 'normal matter' which exists as plasma or as atoms and molecules, in contrast to dark matter) in the universe at the current epoch.[2]"
See http://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium.
Back to the intragalactic medium within galaxy clusters, it seems reasonable to think that less massive galaxy clusters contain similar proportions of hydrogen as more massive ones, but their gravitational potential does not cause the central condensation, heating, ionization & X-ray emissions found in more massive galaxies - and is more difficult to detect. It has been suggested by some that all galaxy clusters contain large amounts of undetected cool hydrogen gas and/or cooled plasma...
The posted question seems to pertain more to the formation of the first stars - and how large hydrogen clouds formed to produce them...
As I understand, universal mass density was much greater in the early universe, although it was more homogeneously dispersed. The ambient density of hydrogen & helium may have been somewhat comparable to that of stellar nurseries...
For a more informed and complete description, please see http://arxiv.org/abs/1305.5178.
Also see http://arxiv.org/abs/1212.0548 http://www.eso.org/public/news/eso1132/ http://arxiv.org/abs/1203.2612 http://www.nature.com/news/nearby-star-is-almost-as-old-as-the-universe-1.12196 and http://arxiv.org/abs/0807.4928.
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