im not an expert but i know a decent amount about a few fields of science and i was just thinking about black holes. some say they could be portals to other universes, and matter that is sucked in by a black hole will become part of the new universe. with that being said, maybe at the end of our universe all thats left is black holes that slowly after probably trillions of years or more suck each other up and combine with any matter and energy left in the universe, and if all matter and energy are reduced to its simplest forms then on the other side of this super black hole there will be another big bang. And before you say "how can it cause a big bang unless all the energy falls in at once", the closer you are to a field of gravity the slower time passes. So all things approaching massive fields like inside black holes might slow down so much they pretty much freeze in time and combine with all future energy getting sucked in afterwards.