Some people might be startled, in disbelief or baffled by just seeing the title of this post, but I actually have more evidence than you'd think.
First of all, in this shot of Lars' Homestead, where Luke is supposedly gone having run away to go find R2, if you look closely, you can make out a transparent figure of Luke through his doorway.
Second, Luke continuously gets into situations where he should easily die, but somehow manages to remain alive. So my theory is that he already was dead so there was no way for him to die again. Some examples of those situations: When he got tugged underwater long enough to easily drown in the trash compactor, when he got frozen in ice (giving him frostbite), getting whacked by the Wampa and then going unconscious because of the cold, and third, plummeting down a giant tube and falling onto a metal pole which he hung onto, waiting a few minutes to fall again far into a metal ship.
Thirdly, in comics between The Return Of The Jedi and The Force Awakens, Luke shows up, dead. Then, he reappears in The Force Awakens, alive. That would also explain why he went missing because he needed to find or forge a new body to use as his own which would be why he looked so noticeably different in The Force Awakens.
You might point out though, that to be a force ghost you would have to know the force. So my theory for that is that he used to be the most powerful Jedi in the galaxy, but got killed by the most powerful Sith in the galaxy, Darth Vader. And in dying he got a type of amnesia in which he forgot that he was even dead and forgot how to use the force and things like that. That would also explain why in Episode IV, A New Hope, Vader was in such a state of disbelief when The Emperor announced that Luke was alive and aboard the Death Star.
I hope that I made compelling arguments and I want to know what you think in this poll