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.