What do you mean by "a scientific theory cannot be proven"?
Honest enough question.
Take something pretty simple, like say Hess' law.
It says that the energy change in an overall chemical reactions is equal to the sum of the energy changes in the individual reactions that make up that change.
But this is a very important point here:
Hess' law is true under all known circumstances.
That does not mean that it has been or can be PROVEN that from the beginning to the end of time, in all parts of the universe under all possible conditions that Hess' law will still always hold true.
Maybe it does, maybe it does not. We will never know.
Ok so far?
The ToE is supported by all known evidence that is applicable. Said support comes from all of fhe hard sciences.
No contrary evidence has ever been generated, there is no contrary data from any scientific inquiry anywhere.
That being the case, the ToE does very well, as a theory.
It cannot be proven that there lurks nowhere the precambrian bunny to spoil it all.
It is a vanishingly small chance that anyone will ever find an exception to Hess' law. Likewise with ToE. Or some several other laws and theories in science.
Talk of "proof" tho, is for alcoholic spirits, and math. In science, one does not speak of proving things.
(added later...yes, that IS often said, but apparently hardly often enough!)
Does that explain it?