Note before reading: this is my first theory, and as such will have a lot of flaws/ doesn't work with laws of physics/ doesn't work at all. So feedback and corrections will be much appreciated! Also I do not know all the theories out there, and this may be similar to a preexisting theory, which is not my intention.
So this is a theoretical model of a cycle involving two universes. Imagine a 3D cylindrical infinity symbol, with the middle two lanes(?) connecting. At the center (from a top or bottom angle) of the overlapping lanes is a indefinitely tall line that divides the overlapping part into four sections, each connected to a side. A universe starts in one section, being created and expanding, until it leads to the other connected section, which closes, representing the universe ending and everything being destroyed. On the opposite section of the destroying section, a second universe is created (note: the second universe was created at the same time as the first universe). This universe has the same path as the first, being created and then destroyed. Both universes can last any years below infinity, as long as the opposite universe lasts the same time. The line in the center and the space outside the model represents nothingness, the space between destruction and creation, with a infinite volume. Both universes end because the other must begin and both risk chance of colliding if it one both aren't destroyed. Space time can exist inside the cylindrical tubes, being a finite rotating cycle within (imagine a cutaway of the model, with space time going around the circle until the end of the universe).
If you could not understand this, the file linked might make it easier.
model drawing.pdf