We can only observe the observable universe

This is a nice tautology. You are saying "we can only observe the observable universe." This is true. 

You admit that we do not observe the universe that we cannot observe. This is true too. But we can only know what we can observe. Therefore, we do not know what we cannot observe. We do not know the universe as a whole because we cannot observe the universe as a whole.

Physicists know that they don't know the universe as a whole but they invent a creation myth called the Big Bang which is defined as the beginning of the universe as a whole. How do you know the beginning of the universe as a whole if you don't know the universe as a whole?

Cosmology is a hoax. Physicists first need to learn the difference between the words "cosmos" and "the universe as a whole".