Think of the universe like a loaf of raisin bread. As the dough rises, the raisins get father apart. The volume of the universe is expanding along with galaxies traveling at their speed. So it is a combination of both. The raisins (galaxies) are moving apart, but the space they sit in is also moving apart, which allows for the galaxies to get very far apart from one another, and essentially overcome speed of light barriers without actually breaking any physical laws.
The volume of space has expanded many billions of light years across in 13 billion years.
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