There is nothing about this outside the movies, to my knowledge.
Most likely the explanation is exactly what Qui-Gon believes to be true, that a midichlorian is Anakin's father. This is the explanation that I still believe to be true.
Another belief that has been speculated about recently, is
*episode 3 spoiler warning*
that the Sith may have manipulated the midichlorians in Shmi to create Anakin. So they believe that either Darth Sidious or Darth Plagueis are essential Anakin's "father".
At this point, those are the only logical explanations, considering the fact that Anakin's raw force potential greatly excedes any other force user alive, it seems unlikely that any of them would father him.