I come in a little late on this, but there is an important point that got missed in the discussion so far that must be mentioned.
Taz estimated the force on a hydrogen atom on the OUTER RIM of a gas cloud. In other words, he starts off with an extremely inhomogeneous gas distribution in space, where in one region of space is concentrated (astronomically speaking) all the matter, which is surrounded by vast regions of perfect vacuum.
The original question however was: How can we get from a uniform gas distribution to such an extremely clumpy system in the first place? This puzzling question has not be addressed here yet, at all.
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