One of the biggest revolutions in our scientific understanding of the Universe occurred at the end of the last century, when we discovered the existence of dark energy. Prior to that, the Big Bang — where the Universe began from a hot, dense, rapidly expanding state that cooled and spread out over time — was the starting point for the ultimate cosmic fight.
As great as the initial expansion was, all the matter-and-energy in the Universe implied there was a tremendous gravitational force working to pull everything back together. It was taken as a given that there would be three possible fates to the Universe: