Clearly the Dark Side cannot exist on its own since users attempt to tip the scale completely in their favor to achieve total galactic domination. The Sith, Empire and First Order have proven this. So the only solution is to put an end to both sides and allow individuals in the middle of the spectrum to maintain the balance.
This is what Luke means. When Rey says she saw light and darkness, Luke responds by basically saying there’s so much more to it. He sought out the first Jedi temple not to hide but to learn from the past. Something no one can seem to do (yeah, I’m looking at you, First Order, with your Death Star on ‘roids).
I theorize that Luke learned the truth through studying codes and scripture he found at this first temple. (The following is all speculation but all based on straight canon and facts.) Inside, he discovered the writings of the first Jedi, the journal of the Whills, which was canonized in the novelization of “The Force Awakens.” Inside this journal is a poem explaining what must be done to attain balance. The Guardians of the Whills protected the temple of the force-sensitive kyber crystals, which were used to power lightsabers and later the Death Star. The poem was short and simple, reading as follows:
“First comes the day
Then comes the night.
After the darkness
Shines through the light.
The difference, they say,
Is only made right
By the resolving of gray
Through refined Jedi sight.”
This poem (I theorize) is what was written on that piece of parchment with the Jedi insignia seen in the trailer.
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It speaks of the gray, which is an old concept that speaks of Force users that do not conform to the light or the Dark Side. They understand the balance of the two and harness both sides. These legendary individuals were known to be the wisest and most powerful of all Force users.
Luke has become one of these beings, which is why he is adorned in all gray. He wore white in episode IV and black in VI as he explored both sides of the Force. Now wearing all gray (mixing white and black, wow) he is presented as having struck a balance. This is how Luke will train Rey and how they will overcome the First Order.
There will be death and destruction before this can be achieved. But once it is, true balance will be struck.
Still don’t buy into the whole no Jedi premise?
Consider the following: The Empire reigned supreme for approximately 22 years before the Rebellion was finally victorious. The Sith were destroyed at last and Luke tried to raise a new order of Jedi and, by doing so, opened up the possibility of someone being seduced by the Dark Side or a lust for power. Ben Solo did exactly this, and the First Order took off.
By allowing the Jedi to remain, there is always a temptation to the Dark Side that can overwhelm those not well-versed in its teachings. The gray Jedi, being trained in both sides of the Force, cannot be seduced since they practice both sides. You already control your darkness so it can never control you.
I’ll cut my rant short there. But if you have any questions or want to challenge any of my points, please feel free to reach out.