In the Original Trilogy, the Force is said to be an energy field that binds the galaxy together which only a few individuals are able to master control of. Characters speak of the Force in vague terms, which roots the Force in zen-like mysticism. In The Phantom Menace it is revealed that Force powers are actually the product of microscopic organisms called midi-chlorians that live in the cells of all living things. The amount of midi-chlorians in someone's cells can actually be measured to determine that person's connection to the Force, something that nobody bothered to do in all of the Original Trilogy because all Force users were able to simply sense each other's abilities. So in the Phantom Menace instead of Jedi Qui-Gon Jinn meeting Anakin and saying, Wow, I sense a really strong connection to the Force in this kid! like Obi-Wan, Yoda, and Vader were able to do with Luke, Qui-Gon uses a blood test to test Anakin's midi-chlorian levels. The Prequel Trilogy even implies that Anakins high number of midi-chlorians somehow cause him to be conceived, which must complicate paternity tests in galaxies far, far away. It is a convoluted scientific explanation for something mysterious that never needed an explanation in the first place. Furthermore midi-chlorians are rarely referenced again throughout the entire Prequel Trilogy, making it even more unnecessary.