The expansion of the Universe is not the applicable over all scales. It only applies at scales large enough that local forces aren't strong enough to overcome it. Your 2 year old son is not expanding because the bonds between the molecules holding him together prevent it. The solar system is not expanding because the gravity of the Sun prevents its. Our galaxy isn't expanding because its own mutual gravity holds it together. Even our local group of galaxies is gravitational bound together against the expansion of the universe (Evidence of this is the fact that our neighboring galaxy of Andromeda is actually decreasing its distance from us). It isn't until you get past the scale of galaxy clusters that these clusters are far enough apart that their gravitational attraction to each other is not enough to stop them from being pulled along with the expansion of the universe as a whole.
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