Im not sure if this idea has been put forth before, but I was watching Neil DeGrasse Tyson's "Cosmos: A Spacetime Odyssey" and they were talking about how dark energy affects the expansion or contraction of the universe over time. It occurred to me that if the universe was, towards the end of its life to contract to a central point of immense density, that could open up a black hole (or the centre of the universe could be a black hole to begin with). With the Theory of Relativity, Black Holes could theoretically be gateways back in time. Therefore, the entire universe could contract to an immensely dense size, be transported back in time to the start of the universe via black hole, then explode due to its immense density, causing the Big Bang in the first place. Of course, I dont have much of an idea how science works, hence why this isnt in r/askscience, it just occurred to me and i wanted peoples opinions.