[sigmoid10]

That's sort of what's happening. If you imagine space as a rubber sheet with the galaxies pinned to certain places, something is pulling the sheet apart, making it seem like the galaxies are accelerating away from each other. We know the math behind this mechanism thanks to general relativity (it only needs a certain energy density for empty space), but we don't know what causes it. So we just call it "dark energy." Without it, the original expansion of the universe caused by the big bang should be slowing down due to gravity. Dark energy causes it to accelerate instead.