Proof or it didn't happen. That's how it goes. Regardless of how democrati Iran may or may not be, regardless of how much you might hate or like either canditate - if one wins, and oyu can't prove fraud, you deal with it.
Describe unto me a reasonable mechanism for counting eleven million votes in two hours. All such votes are on blank, handwritten cards; all must be checked for voter number and who was voted for, in addition to anything else that may or may not be on them (I have no idea if Iran's presidential election occurs at the same time as any parallel local elections, but this seems unfortunately imminently reasonable).
You cannot. Even if you could, it would not match the election mechanism in Iran, which is small enough they would have to be processing these votes at quicker than one a second to possibly make such a thing work.
And this mechanism we have just described is only for the margin by which the ruling group claims to have won, not even the whole country.
The entire voting system in Iran might as well be
designed to be abused, but in this case they did not even make a pretense of a natural vote.