1 possible future is The Big Crunch. This theory assumes that the average density of the universe will be enough to stop its expansion and begin contracting.
2. Some theorists state that the expansion of the universe will go on forever, and if that happens, the universe will cool as it expands, and then it will become too cool to sustain life.
3. The Big Rip. The universe may keep expanding, as I mentioned before. But as it expands, the gravity may not be able to hold all the planets together, as it is now. Galaxies would tear apart first, followed by smaller black holes, planets and stars. Eventually, space would start expanding faster than the speed of light, and atoms would start breaking down into super small particles and everything would fall into nothing.