Depends who you ask
====

There are a lot of different views between the Force, some which have a Light and Dark side, some which don't, and some which have some other view.

The Living Force
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The philosophy of the Jedi Order,  the [Living Force][1] was divided between light and dark, with no state in between. Those who try to walk in the middle are eventually drawn to one side or the other, and the Jedi are always fearful of the dark side.

*Binary dark and light, no in-between.*

The Ones
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Anakin Skywalker encountered [The Ones][2] who are a family of Force users that some believed to be god-like beings who embodied the Force, with the Son being the Dark Side, the Daughter being the Light Side, and the Father being a sort of balance keeping the two in check.

There is some [debate among fans][3] about whether or not The Ones are true embodiments of the Force, or merely a family of immortal beings who were more symbolically drawn to aspects they saw as representative of the Force. It's notable that all three died, yet the Force continued to exist. Regardless, their existence seemed to be relatively unknown, and Anakin Skywalker still seemed to believe in the Living Force, despite having met these beings.

*Binary dark and light, but there is a well-defined in-between "balance".*

Unifying Force (Legends)
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Adhering to a philosophy called the [Unifying Force][4], the Force is one single entity that is neither light nor dark. It was something akin to a deity that did not take sides. Note that there does seem to be some overlap here with the Living Force; Yoda seemed to believe in the Living Force and the concepts of a Light and Dark Side, but believed that visions were neither.

*No concept of dark or light exists.*

Potentium (Legends)
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Adherents to the [Potentium][5] view of the Force believed that the natural state of the Force was inherently good, and that there is no light and dark side. Rather, it is a tool and only the intentions of the users of the force to do evil that are evil. If you want to use Force Lightning all day with good intentions, that's absolutely ok. This view was heresy for the Jedi Order, since it denied that the Dark Side of the Force existed.

*The Force is inherently good, only your intentions can be evil.*

Way of the Dark (Legends)
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The opposite of the Potentium view, the [Way of the Dark][6] asserts that the Force is inherently evil, and that ultimately it trends towards destruction. This takes the view that any concepts of good are irrelevant since you are going against the natural way of the Force.

*The Force is inherently evil, and good is an ultimately meaningless concept.*

The Insidious Force (Legends)
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A view with some commonalities to the Unifying Force, the [Insidious Force][7] states that the Force is like an uncaring, sinister deity. After all, nearly every conflict in the Galaxy Far, Far Away is a result of the Force. It seems adherents do subscribe to the idea that there may be a light and dark side, but it seems to be inconsequential

*The Force is inherently uncaring and sinister, light and dark may exist, but are inconsequential.*

Aing-Tii beliefs (Legends)
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To the [Aing-Tii][8] monks, The Force is neither light nor dark, but rather a spectrum like a rainbow. In other words, you can be more light or more dark, but can also be right in the middle as well. These monks typically stayed on their homeworld, so few knew about this philosophy.

*Light and dark exist, but as two ends on a spectrum.*

Summary
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In Disney canon, it appears that the Force is pretty definitively drawn between the Light and Dark Side, and if there is an in-between, it doesn't exist as a spectrum. In Legends, there are a lot of views that do allow for an in-between, or suggest that The Force is of a different nature.


  [1]: http://starwars.wikia.com/wiki/Living_Force/Legends
  [2]: http://starwars.wikia.com/wiki/Force_wielder
  [3]: http://starwars.wikia.com/wiki/Talk:Force_wielder/Legends
  [4]: http://starwars.wikia.com/wiki/Unifying_Force
  [5]: http://starwars.wikia.com/wiki/Potentium
  [6]: http://starwars.wikia.com/wiki/Way_of_the_Dark
  [7]: http://starwars.wikia.com/wiki/The_force#The_insidious_Force
  [8]: http://starwars.wikia.com/wiki/Aing-Tii