The midichlorians idea was made after the first three movies. It was like a new thing for episodes 1, 2 and 3. Or so I heard.
I like the idea that the Force is not just magic but has a physical component. However, the Force is still something mystical and misterious, because midichlorians only allow to "sense" the Force, they don't generate it or something like that. The force is still a misterious energy that exists in the galaxy.