While the Big Bang has become the quintessential theory for the beginning of the universe, the possible future of the universe is still an open question. While the universe we currently observe is expanding in all directions, who is to say that this expansion will continue indefinitely. And if the expansion of the universe ends, what happens next? While not all theorists agree with this model, one idea is known as the Big Crunch. This theory states that eventually the expansion of the universe will be overcome by the force of gravity. When this occurs, all of the matter in the universe will begin to collapse. Once this process has begun, there is no going back; the universe will continue to collapse until all of the matter in the universe is once again condensed into a singularity, just as it was before the Big Bang. While this result sounds catastrophic, it is unlikely to concern humanity due to the fact that our sun will die far before the universe would begin to collapse. While this idea is still supported by some theorists, many still continue the debate of the fate of the universe.
Source: The Big Crunch