Short answers, pulled from the story.
George Lucas created the concept of the Force to advance the plot of Star Wars in 1977. He finished the fourth and near-final draft on the 1st of January 1976 which trimmed the original concept of the Force of Others to just the Force.
The Phantom Menace from 1999 introduces midi-chlorians as microscopic creatures that connect characters to the Force. Qui-Gon Jinn states that these microscopic lifeforms exist inside all living cells and allow some characters to be Force-sensitive if they have a high enough count.
Dave Filoni said that George Lucas believed a character's intentions when using the Force distinguishes light and dark sides. Their will to be selfless or selfish is what defines the distinction while villains like the Sith draw on the dark side to act aggressively.
Vincent Canby of The New York Times called the Force a mixture of ESP and early Christian faith in his 1977 review. The Magic of Myth compares the sharp distinction between good and evil to Zoroastrianism while other similarities link the concept to Taoism and Navajo prayer.
Scientists are mostly skeptical about a real world explanation for the Force because it implies time travel of information through precognition. Flavio Fenton of Georgia Institute of Technology suggests a fifth force would carry two types of charge one for light side and one for dark carried by its own particle.