Physicists, mathematicians, and scientists can't seem to agree on how the universe will end. In some ways, they can barely agree on how the universe came to be. Nonetheless, most leading thinkers in the scientific field agree that the universe is not an infinite place and someday, it will cease to exist.
The theories run the gamut from a deathly freeze to getting ripped apart, to the universe crunching into itself. As a barely understood aspect of the universe, many leaders in the scientific community view dark energy as a key player in the universe's ultimate demise. Luckily for us, most predictions are in the unfathomable future, billions or even trillions of years from now.
And while it causes us very little real concern today, overall, it's disconcerting to consider that someday, even if it's billions or trillions of years from today, all that is the universe, all that is being, will one day, not be. And there will be no one left to study what once was, nor will there be anyone to remember us or the universe fondly. Someday, there will be no evidence that our universe ever existed.
Unless that is, before the universe expires, human beings figure out how to move to a new universe. And then, billions or trillions of years from today, if humanity manages to survive, our universe as we know it, and everything that ever existed within it, will seem more distant and more foreign than our current understanding of what kicked it all off...the "Big Bang."