Future of Universe

The Universe seems to be expanding at a faster and faster rate due to the presence of the mysterious dark energy. Normal matter and dark matter tend to slow the expansion down.

Therefore, far in the future, the fate of the Universe will depend on how much of each types of energy there is.

Three types of matter-energy in Universe with estimates of relative sizes

If the dark energy dominates, as seems to be the case, then the Universe will keep expanding forever, to end with a Big Emptiness. If our current understanding is wrong and dark matter dominates then the Universe will eventually stop expanding and start to shrink. If that happens then, very far in the future, the Universe will shrink back down to a point no bigger than a full stop. This is sometimes called the Big Crunch. It would be like the Big Bang backwards. We have no way of telling what would happen after that, or even if "after that" has any meaning.

Although it has not been definitely established, there are several lines of evidence which indicate that the Universe will keep expanding forever.