There are many different theories on how our beloved universe will come to an end. One which I will be talking about with you is called The Big Crunch. This scenario is only probable if the over density of dark energy, which causes the universe to expand, is less than -1, which make sit the least likely.
The odd thing about this scenario is that almost all of the events are not tied to time, which means we have no idea when the events will happen, but we do know when these events will happen relative to the size of the universe.
The universe will begin to shrink after gravity finally over takes dark energy. The first event that will happen is when the universe reaches 1/5 it's original size (200 billion light years). All the star stuff in the universe will collect together to form massive super clusters.
At this point, we will be able to reach anywhere in the universe if we go close to light speed, which we wouldn't be able to do if the universe was expanding.
Once the universe reaches about 1 billion light years in diameter the super clusters will merge into one massive galaxy. The temperature of the cosmic microwave background radiation will become 0 degrees Celsius, or 32 degrees Fahrenheit. All planets with water on them will become suitable for life.
By now you might assume everything is gonna be fine, wrong! The universe will continue to decrease in size, causing the CMB to continuously rise. A nanosecond before the universe collapses into it's pre-big bang state, atoms will disintegrate down into sub particles. However, another big bang is possible for it to happen, so the universe might just start all over again.