The Universe was born 13.8 billion years ago but when will it end? Nobody knows for sure but we have some interesting theories. The three main ones are 'The Big Crunch', 'The Big Rip' and 'The Big Freeze'. Based on our current knowledge, the Universe is expanding rapidly and is speeding up its expansion. 

The Big Freeze

In this scenario, the Universe will end when it reaches absolute zero (-273 Celsius or -459 Fahrenheit). In the far future, the gas needed for stars to form will be spaced so far out that it won't be possible for new stars to form leading to the temperature to fall to absolute zero. Black holes will dominate the Universe but will eventually perish as well. The Universe will become dark, cold and barren.

The Big Rip

This scenario suggests that everything including time itself will be pulled apart and torn to shreds. The Universe's expansion rate will increase to a point where it reaches infinity leading to everything we understand and know coming crashing down. Galaxies will begin to deform and rip apart followed by solar systems then planets and finally atoms.

The Big Crunch

The Big Crunch hypothesis states that the Universe's total space throughout its lifetime is symmetrical. It started as a singularity gradually speeding up its expansion until it reaches a point where its expansion stops and it begins to contract. It will eventually shrink back into a singularity. We have absolutely no idea what will happen then. We can assume that after we all shrink back into a singularity that it will be the same as before the Big Bang (if there ever was something before it).

These are the three theories gaining the most traction at the moment but there are many other theories backed by science and other ones backed by absolute rubbish.