If dark energy was not expanding the universe, then gravity would keep doing its job of clumping things together and making ever-larger shapes and structure. But dark energy can’t be denied. So we have two competing effects: having enough time for gravity to clump things into massive shapes but not so much time that dark energy has pulled them apart. At the moment, these two effects seem to be perfectly balanced, which means we live in the perfect age to see the largest structures the universe will ever know.
