Ever since the Big Bang, 13.8 billion years ago, a battle has been taking place. On the one hand, gravity, acting between all the matter in the universe, has been working to slow down cosmic expansion. On the other, dark energy, which permeates all of space, has been acting in the other direction to make the universe expand faster. For several decades, scientists have widely believed that dark energy must eventually win and doom the universe to eternal expansion. But recent results have challenged this view.
