How can space expand between individual hydrogen atoms? [Strange Universe Series, 2011]

We know that the Universe is expanding. We know this expansion involved the apparent stretching of relatively empty space devoid of any galactic clusters. The gravity between objects within a galaxy or galactic cluster seems to inhibit expansion, and they remain at a fixed distance from one another except as influenced by their own relative motion. But we also know that even vast stretches of “empty” space with no large bodies in them contain a few hydrogen atoms per cubic centimeter. Since the tiny gravitational pull of one galaxy on a neighbor in a galactic cluster prevents spatial expansion, why doesn’t the gravitational interaction of the hydrogen atoms keep open space stable?

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Here are the previous Strange Universe Series—2011 questions:

5—Since space is expanding and is linked to time, isn’t time expanding as well?
4—In an expanding universe, how do separate galaxies manage to collide?
3—If the universe is infinite, how big is what it is expanding into?
2—How can we be certain the Uncertainty Principle is certain?
1—How do you envision space in more than 3 dimensions, then rotate it to see what happens?

The entire 2010 Series of 20 questions can be found from here.