When the universe was dense, gravity dominated. As it expands, dark energy gets more and more important. From Ars Technica's article:

"If dark matter gives rise to the gravity that holds the universe together, then dark energy is the counter-force pushing the universe apart. Very early in the universe’s existence, dark matter dominated. Everything was closer together, so its density was higher than that of the dark energy, and its gravitational pull was stronger so the early galaxies could form. But as the universe continued to expand, the dark matter density, and hence the gravitational pull, decreased until it was less than that of the dark energy. So instead of the expected slowdown in the expansion rate, the now-dominant dark energy began pushing the universe apart at ever-faster rates."