Are all of the theories of the Universe based upon the limited section of which can actually be measured?

Answer 1

Theories -- as the word is PROPERLY used in science -- can only be based on rigorous and repeated observations. This is why Big Bang Cosmology is an over-arching THEORY, not just a hunch -- it makes specific, testable predictions; and the predictions made by BBC match what we see. That's why we call universal gravity a THEORY.

HYPOTHESES, on the other hand, can be little more than a hunch. In addition, it is perfectly acceptable to have a hypothesis that makes no testable predictions. Indeed, we can even make hypotheses that are, by definition, un-testable.

As an example, "eternal inflation" speculates that our Universe is just one of an infinite number of universes that exist; our just happens to be one where intelligent life is possible. In other universes, there is no intelligent life getting upset that intelligence can not exist in their universe. Of course, we can not presently prove (or disprove) such universes even exist.

Thus, hypotheses about what (if anything) exists outside the observable Universe almost HAVE to be un-testable. It is hoped that, in the future, people more clever than we are at the present will develop experiments OR mathematics that can rule out OR rule in certain possibilities about what we can never observe.