We don’t know what is causing the expansion of the universe, only that we can observe that the expansion is accelerating.
We assume there is nothing outside the universe “pulling it” so we attribute the accelerating expansion to an unknown force we presently call “dark” (i.e. unknown) “energy.”
We believe there’s an unknown energy called “dark energy” that causes the universe to expand the way it does.
It leads to all kinds of interesting ideas about the big bang, such as the big bang gradually accelerated exponentially causing another big bang infinitely.