For all we know....... the Universe is "never ending" and every so often there's a "Big Bang" caused when the remnants of a dead part of the Universe explode. That is, where large collections of dead stars & planets have come together under gravity in a gigantic (I'd better not say "almighty"?) accumulation of matter which eventually goes off in a "Big Bang" due to the immense pressure (+ heat?).

The ensuing explosion creates a "new" part of the Universe all over again. Big Bangs, for all we know may well happen "all over" the Universe and the part we know as "The Universe" is just "our" small bit of it.