There are many theories of this, some scientific others religious however in my opinion the physical universe that is comprised of matter in my opinion my not end but may keep changing over thousands of years.
There are three main ideas.
1. The universe will keep expanding and eventually everything will be so far apart that everything will dissipate into nothing.
2. The universe will eventually stop expanding and roughly stay the same as it is now.
3. The universe will stop expanding and then start contracting, and maybe cause another Big Bang.
I like the idea of the third one, perhaps the universe is forever expanding and contracting and has already gone through multiple Big Bangs.
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