Luke Skywalker's absence from marketing for The Force Awakens triggered a number of theories, including speculation that he had turned to the dark side, with some theories going as far as to suggest (against all evidence) he was the man behind Kylo Ren's mask. Obviously, none of this speculation came to fruition in Episode VII, as Luke was simply hiding on Ahch-To until Rey discovers him at the end of the movie, not even speaking a line of dialogue. Even so, marketing for The Last Jedi has brought "dark Luke" theories back with a vengeance, especially with a number of posters suggesting he's not simply a straightforward protagonist this time around.The thing is, while it's definitely a tantalizing idea, dark side Luke doesn't quite add up. A Luke that had succumbed to the dark side would have craved revenge and tried to track down Kylo Ren and Supreme Leader Snoke and attempted to kill them. His exile suggests quite the opposite. But that doesn't mean he didn't give himself over temporarily, and the red kyber crystal rumored to hang on a necklace around his neck and the absence of his green lightsaber in the movie's marketing just might be the key to figuring him out.

The absence of his lightsaber is the first intriguing part of this mystery. After losing his father's old weapon along with his hand during the duel with Vader on Cloud City, Luke constructed a new, green-bladed lightsaber. Some fans think that saber was lost when the second Death Star was destroyed, but it can clearly be seen on the young Jedi's belt during the party with Ewoks on the Endor moon, and it also shows up in Shattered Empire, the only canonical post Return of the Jedi story explicitly featuring Luke Skywalker other than The Force Awakens.

The lightsaber's marketing absence in and of itself isn't very noteworthy, seeing as there's a lot of things The Last Jedi still isn't showing, but even Luke's action figures don't come with a lightsaber despite belt loops and hands equipped to hold an accessory of that type. The fact that several posters and some merchandise have featured Luke holding his old blue-bladed lightsaber - the one Rey brings him at the end of The Force Awakens - instead, adds to the mystery. It's possible he simply lost the lightsaber during the 35 years since The Force Awakens, but that's a weird thing to hand wave away. The more likely explanation is that we haven't seen it because it's a spoiler.

While it hasn't been revealed in any images, it was rumored months ago that Luke will wear a necklace holding a red kyber crystal in The Last Jedi. It seems odd that he would wear the crystal from a dark side Force wielder's lightsaber around his neck, so it must be something incredibly personal to him. Unlike Luke's lightsaber, Vader's red-bladed lightsaber was seen tumbling into the depths of the second Death Star before it was destroyed, so it's not a memento from his father, and Kylo Ren's lightsaber is intact, so it probably doesn't belong to him either. There are a number of red lightsaber floating around on the black market and being acquired by a cult called the Acolytes of the Beyond, so there are red kybers around, but Luke doesn't have any reason to wear one of those. The most sensible explanation is that it's because it's his red crystal.

The Ahsoka novel and Darth Vader comics have specified that kyber crystals aren't naturally red. A Force wielder is supposed to have a connection with the crystal they use to make a lightsaber, but the crystals don't present themselves to or cooperate with wielders of the dark side. As a result, when the dark sider assembles their weapon, they have to exert their will upon the crystal, which causes it to "bleed" and turn red. This might be what happened to Luke's old green crystal, which is why they haven't shown that lightsaber in marketing, as it would betray that he had fallen to the dark side.

Luke trained a generation of new Jedi, including his nephew, Ben, who eventually turned on him, killing all the other Jedi. After this, Luke went on a quest to find the first Jedi temple and disappeared into exile. But why? This is nothing like the Luke Skywalker we know from the original trilogy - the one who leaves his home to save a princess he doesn't know, abandons his training to save his friends on Cloud City, gets captured by Jabba to save Han, and eventually turns himself over to the mercy of the Emperor because he believes there's a shred of goodness in his father still. So what could happen to crush the hope and devotion seen in the younger Luke in order to turn him into an old man that retreats to an island, abandoning a budding galactic war, and refuses to even rush to the aid of Han Solo, who's peril he could certainly sense if he could detect the danger to his friends on Cloud City when he was still training? Now, he lives in isolation, thinks the Jedi need to end, and apparently clashes with Rey - if the trailers can be believed.

The puzzle pieces of this collapse are all available in the known story. If Kylo Ren killed the rest of the Jedi, he probably also faced off against Luke. Based on the trailers, it looks like Luke was left for dead, meaning his apprentice had gained the upper hand in their confrontation, possibly by damaging and disabling Luke's lightsaber as we've seen done in a number of lightsaber fights before. Having lost all of his Jedi students, knowing he trained the person that killed them would be a major blow and a major source of pain for Luke. This pain could easily give way to anger and hatred, driving him to seek vengeance on his nephew and Snoke.

But first, he'd need to rebuild his damaged lightsaber. The only problem is, having given into his hatred, he would have caused his crystal to bleed during the reconstruction, turning his green saber red, a symbol that would doubtless be a convicting one for the son of Vader. Shocked by what he'd allowed himself to become, fearing that even if he kills Kylo, he would only be perpetuating a dark cycle, he retreats in search of ancient Jedi truths, wearing his now-red kyber crystal around his neck as a reminder of his failure.

The Luke we see in trailers for The Last Jedi may not have entirely succumbed to the dark side, but he's very likely living in fear and shame of his many failures. This fear, of course, makes him wary of Rey's ability as well, but hopefully she'll return hope to the old Jedi and convince him to join the fight once again. Either that, or she gets captured (again), forcing him to go rescue her, raising his X-Wing out of the sea, and reconstructing his lightsaber, in which case, he may actually wind up with a white blade, a first for a Star Wars movie after Ahsoka Tano was the first known character to purify red kyber crystals in her own sabers in the Ahsoka novel.

There are plenty of unknowns about this movie, giving way to a number of shaky theories, but what makes this particular explanation such a convincing idea is that it's is a single answer that ties together a number of mysteries for The Last Jedi: Why did Luke run to exile, why aren't they showing his green lightsaber in promotional material, why is he wearing a red kyber crystal around his neck, and what happened when Ben turned against him? This single answer also utilizes a swath of Star Wars lore that either doesn't require detailed explanation or can be easily explained with a few lines of dialogue in the movie. The audience doesn't need to know why kybers turn red or who Ahsoka Tano is, just the visual image of Luke igniting a red lightsaber and the blades turning white should be enough of a twist on classic Star Wars iconography to thrill the audience.

NEXT: Is Luke Skywalker The Secret Villain of Star Wars: The Last Jedi?