For cosmologists, the origin of the universe is clear. They can see that galaxies are accelerating away from each other and when they play this motion in reverse, the universe contracts to a single event. This suggests it all began some 14 billion years ago in an event we now call the Big Bang.
By contrast, cosmologists are less clear how it will all end. One possibility is that the expansion of the universe will continue to accelerate, driven by a mysterious force called dark energy. In that case the expansion will be infinite and forever.
