[constantcrying]

I agree that proofs assume for readability that the reader fills in some blanks. I might even go so far as to say that this informality can be dangerous.

Still, the question is, are there "folk theorems", where no proof exist which could be written down fully formal?

[cubefox]

I said there are accepted statements for which nobody can "cite" a proof, not that these statements are not provable in principle.