There is a book just recently released that QUESTIONS whether Muhammad himself ever existed...
The thesis it explores is one that has been too little considered by English-language scholars: What are the reliable historical grounds for crediting the canonical narrative of the origin of Islam?
The author, Robert Spencer shows that there is, in fact, no historical evidence at all that Mohammed ever existed, and that the tales about him were written long after he was dead and were often fabricated to justify one tribe's actions over another. Even the stories about Mecca being an important center of trade appear to be fabricated as its location would preclude it from serving this purpose.
That would of course include, the story of his marriage to the 6 year old Aisha...