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Scientists believe that universe will stop expanding when the matter present inside it crosses a critical threshold. If the amount of matter present crosses the critical threshold then the universe will first slow down and then finally stop.
Now What?
As per scientists as matter has increased so has gravity. So, gravity will start pulling everything inwards. Universe will now start contracting. It will start getting denser as well as hotter as it keeps on contracting further. Eventually it will collapse into singularity. Scientists also refer to this event as The Big Crunch i.e. reverse of The Big Bang.
As per one more research, Dark energy is primarily responsible for the expanding universe. As not much is known about Dark Energy it is difficult to predict its effect on a contracting universe.

Reference:
http://www.bbc.com/earth/story/201506...