A flat universe could only mean an infinite universe because each dimension exists on a plane which continues to expand at an infinite rate.

This is wrong. A cylinder is flat, for instance...

There is a theory that exists known as the theory of inflation

There are several inflation theory as we still don't know exactly the mechanism behind it.

However, recent finding suggest that we are not alone and we live in a multiverse.

Please show me these findings... I may have missed them...