Space isn't an empty vacuum, but that's tangential. The expansion in this context is metric explanation, which just means things are getting further and further apart between two points due to space "expanding" between non-gravitationally bound objects. The explanation for "why" is sort of "we don't know". AFAIK, the leading theory is a combination of momentum / inertia from the big bang and acceleration from dark energy. Which we also don't know a lot about.
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