A reason based in the scientific process.
Reasons, in and of themselves, are not based in the scientific process. The Universe is what it is. Its operation is based in itself. It would be what it is whether or not primates or any other organisms anywhere developed science. The action of a cell is not *based on* the microscope - the microscope allows you see the actions of the cell.
The scientific process (which is a glib phrase, but I'll allow it for the moment) is a method for *finding* reasons. Science is merely a set of tools and processes we mere mortals with our limited sensory apparatus and cognitive quirks have to finding out what the Universe is and how it operates.
Some things such as the big bang cannot be proven in any way.
Depends what you call "proof". In the same sense as a logical mathematical proof? No, but then nothing outside mathematics can be so proven, and no practical scientists claims to prove anything in that sense. In the more colloquial sense of having tons of evidence for it, and making predictions which turn out to happen in reality, however, science does pretty well.
You cant test that theory for truth. You can only look at how things are working (or we think they're working) in the universe and see if it contradicts the theory or not.
And, if the action of the universe contradicts the theory, we know the theory is wrong, now don't we? If the action of the universe *fails* to contradict the theory, and the theory clearly, measurably, and repeatedly predicts what actually happens in many ways, then why shouldn't we figure we are onto something?
And, you can do slightly more than what you say above. A good working theory has the following characteristics:
1) It is consistent with what you already know - all currently known phenomena are covered by the theory.
2) It is consistent with or predicts phenomena that are not covered by other theories.
For the basics, Einsteinian gravity reduces to Newtonian gravity. However, Newtonian gravity doesn't predict the precession of the orbit of Mercury, doesn't properly predict the lensing of light around the Sun, and doesn't handle gravitational redshift of light. Einsteinian gravity does all those better than Newtonian mechanics does. And it isn't that Einstein just cobbled together something that predicts the results of known experiments of his time - there's tests that he couldn't make in his own time, that have been done since - when a theory predicts phenomena that its own creator didn't consider, and fits old phenomena to levels of accuracy and precision unavailable to in the time of the original formulation, that's rather more than, "We can see if the universe contradicts the theory."