Idea I have… Since the Big Bang, the universe has been running down from very low entropy and high energy state to a very high entropy and low energy state. Energy cannot be created or destroyed, so all that energy will be converted to mass. The universe is currently supposedly expanding at a growing rate. Perhaps there will be a time when that growth slows or even reverses? Some force like gravity may slow the growth and start pulling things back into the core of the universe where there would be another Big Bang. Maybe the universe just cycles between Big Bangs followed by high energy expansion, then billions of years later it starts to contract again.