Midi-chlorian/Legends

Midi-chlorians were microscopic life forms thought to exist inside the cells of all living things. They were not, however, the Force themselves: they were only a link to it, acting as a form of sensory organ through which it could be perceived. Midi-chlorians enabled the perception of the Force just as having eyes allowed people to see light or ears allowed them to hear sound. If an individuals had enough midi-chlorians in their body they could use them to communicate with the Force. In essence, midi-chlorians were the connection between a being’s mind and the Force, enabling certain sentients to intentionally manipulate it.

A high midi-chlorian count was usually an indication of Force-sensitivity, meaning that the creature had the potential to become a Jedi. The Jedi Order was always on the lookout for Force-sensitives, whom it would bring to the Jedi Temple and induct into the ways of the Force. To further this goal, the Jedis learned how to quiet their minds and focus, so they would be able to hear the midi-chlorians that lay inside of other creatures. Once a being was suspected of having a high midi-chlorian count, a simple blood test could be administered to determine a definite amount.

Midi-chlorian research
The Rakata were thought to have experimented with medically transferring midi-chlorians into other beings in an attempt to transfer Force-sensitivity. They supplanted the genes responsible for midi-chlorians from Force-sensitives into their own genetic code to try and help their race 'remember' forgotten Force abilities. It is not known whether these experiments were of any success, as the Rakata believed it would still take many generations for the experiment to complete.

Millennia later, the Dark Jedi Desann managed to artificially infuse the Force to non Force-sensitive individuals by using special crystals. Desann referred to these people as the Reborn. Whether his method had anything to do with midi-chlorians remains a mystery, but it is thought that either the crystals caused the midi-chlorians to multiply or somehow 'reinforced' the ones that already existed, producing a totally artificial Force-sensitivity.

Behind the scenes
The concept of midi-chlorians was introduced in Episode I. Some fans of the saga saw midi-chlorians as hard science to the mysteriousness of The Force and disliked the new concept.

The cause-and-effect relationship between midi-chlorians and Force-sensitivity seems to be a matter of debate. In The Phantom Menace, Qui-Gon Jinn states that users sense the Force through midi-chlorians; however, some argue that he may have been giving Anakin Skywalker a childish explanation, and that the midi-chlorians may actually be a simple indicator rather than the actual method of communication.

Despite how it might first appear, the existence of non-living yet Force-enhancing objects such as rocks and crystals, does not seem to contradict the midi-chlorian explanation. Midi-chlorians are simply a way for organics to perceive the Force, and thus a non-organic object can be strong with Force without perceiving it, just as a light emitting object does not necessarily have eyes.

The existence of some non-organic beings which are occasionally Force-sensitive (such as the Shards and the Tsil) does pose more of a problem, though an explanation may yet appear.

It is possibile that while midi-chlorians provide a way to communicate with the Force, they are not necessarily a measure of Force ability as a whole. This idea is supported by the fact that both Anakin Skywalker and Yoda, the two Jedi with the highest known midi-chlorian count in the Jedi Order, seem to be the most prone to visions received through the Force; or at least more so than others of similar Force ability (such as Mace Windu).

Appearances

 * Star Wars Episode I: The Phantom Menace
 * Star Wars Episode III: Revenge of the Sith