A rhetorical question is one to which the answer is so seemingly obvious that it doesn't require an answer, but it is still a question and thus requires a question mark. (Of course, if it is an exclamatory question, it's OK to use an exclamation mark instead.)
For example,
"Who does not know that the USA has been involved in Iraq?"
Faced with such a query, no one is very likely to say, "I don't."
Or, said to a known liar, "We all know you're as honest as the day is long, don't we?"
The whole point of such questions is to make the idea contained in them stronger than a simple statement would be.