You usually cannot have a preposition directly before the conjunction 'that'.
"You will find out that you won the election."
Notice in the above statement "find out" is a verbal phrase. "Out" cannot really mean anything by itself.
A quote from Jespersen, Philosophy of Grammar, Pronouns:
... the impossibility of having a preposition before 'that': "the man that you spoke about" as against "the man about whom you spoke" ...