The universe will end when the very last sun consumes the last of its fuel and goes out, when the last piece of rock collides for the last time, and when the last thing that is going to happen, happens. After the last intelligent life expires, there will be no need for time and it too will cease to exist. Maybe all the black holes will converge into one gigantic black hole and consume the universe and then reach singularity and produce another Big Bang and start a universe all over again for the 10,000,000th time. Who knows. The only thing for sure is there ain't anybody going to be around to witness it, and I will be out of town that day.
Some talk about the rubber band theory which states that the universe is constantly stretching out in all directions(there is substantial proof). Once the universe reaches the point that it cannot stretch any farther, it will snap back in a flash and collapse within itself, thus creating another big bang and possibly another universe. This is only a theory, and we may never know what may or may not happen.
The thing is the universe is not only expanding but it is accelerating. There are some who think that the universe will eventually burn out all of its hydrogen and become an extremely large inert could of dust. There are others who think there will be a big crunch in which the universe will recede and everything will come back together, all matter will be compressed into a singular massive molecule which will cause all of the atoms to smash themselves simultaneously converting everything in an instant into energy, which at that point gravity would become a null value, and then there would be another big bang. One of the problems with this theory is that the gravitational influence of the mass could not pull the universe back together again because everything is too far apart and is continuing to spread out at an increasing rate. Dark matter is being investigated as a possible solution to this issue, but these theories are still in their infancy. I hope there is a big crunch, but the evidence seems to support the universe becomes inert might be the likely scenario.
The Universe is expanding in all directions at an accelerating speed. The simple extension of that observation is that eventually there will be no stars in the sky and even our individual atoms will fly apart. So, in the end, there will be nothing left. Right?