After watching the movie yesterday and processing dialogue, possible motives, probabilities and such I'm going with Rey being Luke's daughter.
I know wow that came out of left field right?

I haven't read many theories but I'm sure it's a popular one.
But here's the rest of it: Rey's mother/Luke's wife was at the Jedi academy when it got hit by Kylo and the Knights of Ren and died at their hands.
Reys mother was also a Jedi or at least a student. This is the explanation for Reys accelerated progression and strength in the Force,she is the offspring of two Jedi.
Think about it: Ben is the product of just just one parent strong in the Force but Rey has two. Remember the Jedi in the prequels were forbidden to marry and have offspring for what ever reason so we don't really know in movie canon what would happen. But obviously it did happen in fan fiction with Mara Jade marrying Luke.
But I don't recall that that made Luke's offspring more powerful than Leia's. Well I think this is now the official answer from Kasdan on what happens when two Jedis get busy.
As for why Han and Leia don't know her? Luke probably thought it best that the Jedi train in solitude and away from Republic and Resistance oversight and not have official ties. (One of the few lessons learned from the Chancellor Palpatine era). Luke of course wanting to protect his Uber Jedi daughter from baddies pulls a Kenobi and even sends her to a sand planet (at least Kasdan is smart enough to realize putting yet a third Skywalker to grow up on Tatooine is beyond the pale for even the most die hard StarWars fan to swallow) Still pretty sad Luke would do that to his only(?) child seeing how he didn't really like it himself. Von Sydow was probably the one who was watching over her from a distance and of course had to die like Owen and Beru. No coincidence that Sydow has the missing map piece and is revealed at the right time to set things in motion.
Anyway that's my theory and I'm sure someone else had it before me. But I havent read many other articles or come into this thread much before seeing the movie yesterday so as to avoid spoilers.