Universe or Multiverse?

With all the discussions about Flat Earth it has made me do some more research and thinking.

A big question is do we exist in a Universe or a Multiverse. The Universe would include all space and matter that we have access to. Is it possible that there is other space and matter that we do not have access to?

Further, outside of the debate on the shape of Earth what is the shape of our Universe? Is our Universe flat as well or what shape does it take?

This is a worthy series of lectures MIT has put together. Even the Professor admits that there is much that can be debated and much that is unknown and based on assumptions.

https://ocw.mit.edu/courses/physics/...-a-multiverse/