"Your eyes can deceive you. Don't trust them. " - Obi-Wan Kenobi
Remember our first lesson taught by Master Kenobi? Well pay attention to what The Force Awakens is telling you because the old dead fossil is still taking us to school.
So we are to believe that in the 32 years since ROTJ a giant scary hologram named Supreme Leader Snoke who seems to be powerful with the Darkside of the Force has managed to build the First Order and seduce Luke's pupil Ben Solo to follow the dark path as well. It's the Wizard of Oz Darkside style. They even cast the amazing Andy Serkis to make it more credible.
Supreme Leader Snoke is Leia. She broke bad after becoming Luke's first pupil. She is pretending to be Snoke.
"Search your feelings. You know it to be true." - Darth Vader
Presumably Leia was the only other person who could be as powerful as Luke.
She watched as her planet and family were destroyed.
She fell in love and pushed out Twins, both are not exactly recommended for Jedi.
Now she has two armies and everyone fooled except Luke.
Luke knew he lost his sister to the Darkside and couldn't save Kylo but could save his niece Rey. So he hid her and went exile knowing only the child could redeem the parent.
The first clue in the film is in the first scene with dialogue as Max Von Sydow and Oscar Isaacs discuss how great Leia is.
Both Snoke and Leia are desperately searching for Luke to find Rey.
It's perfect. It's the "I am your father" moment for episode 8.
"If you will not turn to the Darkside then perhaps she will." - Darth Vader