I thought the most popular theory at this point in time is an extension of the Big Bang theory; that the universe is still expanding from the 'bang' and that when it is done expanding is will retract, reversing back into a small amount of space meaning everything will get crushed into a dense area... and then a 'Big Bang' will happen again and the universe will expand again etc.
This is of course in layman's terms and chances are I'm not correct but I remember hearing all about this at an astronomy talk I went to about a month ago.
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