There is a unified reality to life and it exists everywhere in the Universe. And it controls things but you can also control it. That's why I split it into Personal Force and Cosmic Force.
The personal Force is an energy field created by our cells interacting and doing things while we are alive. When we die, we lose our persona and our energy is assimilated into the Cosmis Force. If we have enough Midichlorians in our body, we can have a certain amount of control over our Personal Force and learn how to use it, like the Buddhist practice of being able to walk on hot coals. The Jedi will train you to connect to your Personal Force, and then to connect to the Cosmic Force. You don't have much power to control the Cosmic Force, but you can make use of it.
The Whills are a microscopic, single-celled life-form like amoeba, fungi, and bacteria. There is something like 100000 times more Whills than there are Midichlorians, and there are about 10000 times more Midichlorians than human cells.
The only microscopic entities that can go into the human cells are the Midichlorians. They are born in the cells. The Midichlorians provide the energy for human cells to split and create life. The Whills are single-celled animals that feed on the Force. The more of the Force there is, the better off they are. So they have a very intense symbiotic relationship with the Midichlorians and the Midichlorians effectively work for the Whills.
It is estimated that we have 100 trillion microbes in our body and we are made up of about 90% bacteria and 10% human cells. So who is in service to whom? I know this is the kind of thing that fans just go berserk over because they say, "We want it to be mysterious and magical", and "You're just doing science."
Well, this isn't science. This is just as mythological as anything else in Star Wars. It sounds more scientific, but it's fiction. It's saying there is a big symbiotic relationship to create life, and to create the Force, but if you look at all the life-forms in the universe, most of them are one-celled organisms. I think of one-celled organisms as an advanced form of life because they've been able to travel through the universe.
They have their own spaceships - those meteorites that we get every once in a while. They've been living on those things for thousands of years, they've been frozen, unfrozen, and can survive almost anything. The one-celled organisms have to have a balance. You have to have good ones and bad ones otherwise it would extinguish life. And if they go out of balance, the dark side takes over.