As with most things there are a few theories. This is no different. First, there is the universe will expand until it stops and than implodes. The next is that it will expand forever ending in a Big Emptiness. Finally, the universe will just slowly deteriorate.
The first theory is expansion. If there is a lot of dark matter between the galaxies than the universe will expand but will eventually stop. At that point, the universe will shrink into a very small space. This is called the Big Crunch. We are not sure what will happen to us. If whether there will still be an “us”, or if we will be lost forever.
Deterioration is exactly what it sounds like. The universe will start to wither away. First, most of the stars will either blow up and collapse or they will degrade to white dwarves which are the beginnings of a star. Trillions of years after that the only things left will be black holes. Finally, trillions of trillions of years later, the black holes will dissipate. All that is left will photons, neutrinos, electrons, and positrons. Basically, the universe as we knew it is gone.
Of course these are only theories and none of these are 100 percent correct. Until we are more advanced in society, we will never know what will really happen.