Snoke is a Force Spirit
One major unanswered question at the end of Star Wars: The Force Awakens regards the identity of Supreme Leader Snoke, the ruler of the First Order. Who is he, and what does he want? Is he a new player on the galactic scene, or an old villain returned?
I put it to you that he's a Force Spirit — the spirit of a very familiar villain.
It is well-established that some Jedi Masters are capable of living on as Force spirits after death. The ﬁrst known Jedi to accomplish this was Qui-Gon Jinn. An unorthodox Jedi, Jinn would sometimes deliberately break the Jedi Code, and his studies of immortality may not have been sanctioned by the Jedi High Council — of which he was not a member, possibly due to his anti-authoritarianism and unpredictable attitude. During Jinn's lifetime, shortly before the fall of the Republic, Jedi tradition held that eternal life (the ancient goal of their rivals, the Sith) was impossible.
Although Jinn was killed by Darth Maul during the Battle of Naboo, he returned several years later as a disembodied voice, attempting to convince a young Anakin Skywalker to stop committing a massacre in a Tusken settlement. He would also occasionally talk to both Obi-Wan Kenobi and Yoda. In most of his interactions with the living, he appeared as a disembodied voice, with only a single, highly controversial sighting of a physical manifestation.
Jinn's apprentice, Obi-Wan Kenobi, displayed a much more complete mastery of the ability. When Kenobi was killed during his ﬁnal duel with Darth Vader, his body physically vanished and he immediately began guiding his apprentice, Luke Skywalker. He helped him make the fateful proton torpedo shot that destroyed the Death Star during the Battle of Yavin, and made regular appearances as a shimmering apparition of his former physical self, looking as he did just before his death.
The Jedi Master Yoda also lived on as a Force spirit, after dying peacefully of natural causes in his exile on Dagobah. Like Kenobi, Yoda's physical body vanished, and he would reappear as a shimmering apparition or a disembodied voice to offer guidance to Luke Skywalker. Like Kenobi, his spirit form retained the appearance of his physical form shortly before death — old age included.
Force spirits can interact with people they never met during their physical lives. Both Yoda and Kenobi would speak (as disembodied voices) to the untrained Force-sensitive Rey in a vision, decades after their deaths.
The Fall of Anakin Skywalker
When Anakin Skywalker / Darth Vader murdered his former mentor Obi-Wan Kenobi, he was clearly unaware that Kenobi had mastered eternal consciousness. In the ﬁnal stage of their last duel, Vader mocks how old age has weakened Kenobi's powers, and when Kenobi calmly declares that he will become "more powerful than [Vader] could possibly imagine" if he is killed, Vader pays it no heed. After Kenobi's physical form vanishes, Vader pokes at the empty Jedi robes in confusion, clearly uncomprehending as to what has just happened.
Vader's fall to the Dark Side was itself prompted by the search for a means to defeat death. In the ﬁnal days of his life as Anakin Skywalker, he had precognitive visions of his wife, Padmé Amidala, dying in childbirth. At that point in history, the Jedi order officially believed eternal life to be impossible, and their research into cheating death using the Force was restricted only to Masters of the order. This was consistent with the stoic Jedi philosophy, which promoted the virtue of "letting go of everything you fear to lose" (as expressed by Yoda) — including loved ones and even life itself. In contrast, the Sith Lord Darth Sidious (later Emperor Palpatine) tempted Anakin with tales of a legendary Sith Lord who was able to cheat death. Although Sidious never claimed that he himself had that particular ability (which he stated was a power "only one" Sith Lord had achieved), he promised that he and Anakin would discover the secret together. Thus, Anakin Skywalker became Darth Vader, Sidious' apprentice. As Vader, he would go on to become an instrumental ﬁgure in the ﬁnal purge of the Jedi order and the subjugation of the galaxy under the Empire. During his ﬁrst battle with Obi-Wan Kenobi, he was seriously injured, and most of his body was replaced with mechanical prosthetics.
Anakin's ﬁnal days in the Jedi order, and the nature of Sidious' temptation, explains why he paid no heed to Kenobi's threat, and why he was confused by Kenobi's body disappearing: He was unaware that any Jedi had the ability of eternal consciousness. He himself had experienced a Force spirit — the spirit of Qui-Gon Jinn — communicating with him, but since he wasn't aware that the ability existed at the time, he may have disregarded it as a purely psychological phenomenon (eg. his subconscious producing the voice of Jinn as a "voice of conscience"). Furthermore, Jinn's incomplete mastery of the ability may have made his influence on Anakin too unsubstantial for Anakin to realize what he was experiencing.
During the Battle of Endor, Vader ﬁnally turned against Sidious: Love for his wife prompted his fall to the Dark Side, and love for his son prompted his return to the Light. Witnessing Sidious torturing Luke (who, assisted by the spirits of Yoda and Kenobi, rejected Sidious' temptations), Anakin hurled Sidious into the reactor shaft of the Death Star, killing him — but not before being mortally wounded by Sidious' Force lightning. In their ﬁnal farewell, Luke removes the mask of his father, revealing his scarred and withered form, his face ruined from his injuries during his ﬁrst battle with Obi-Wan and prematurely aged by the corruption of the Dark Side. Anakin died shortly afterwards.
After his death, Anakin displayed a mastery of eternal consciousness similar in extent to that of Yoda and Obi-Wan, manifesting as a shimmering apparition. However, unlike Yoda and Obi-Wan, he appears as his younger selfI regard the ﬁnal edition of "Return of the Jedi" as canonical events., before his fall to the Dark Side. His body is intact, and he appears as he did during his ﬁnal time in the Jedi Order — in his twenties, unscarred, with a full head of hair. It is a point of controversy whether his body disappeared at the time of death: Vader's armoured suit is burned on a funeral pyre by Luke, but it is possible that Luke simply performed a ritual cremation of the suit for emotional reasons, without Vader's body in it.
The Return of Darth Vader?
Unlike Qui-Gon Jinn, Obi-Wan Kenobi and Yoda, Anakin never studied eternal consciousness during his time as a Jedi, apparently not even being aware that the ability existed. His puzzlement with Kenobi's body vanishing is evidence that at least as late as the Battle of Yavin, he didn't understand it. One possible explanation for how he achieved the ability is that it somehow came naturally to him (his prodigious Force powers are known to have surpassed even those of Yoda), or that perhaps Qui-Gon, Obi-Wan and Yoda (all of whom had known him as Anakin) stepped in to help him live on, during the last moments of his life. But another, more disturbing possibility is that Sidious ﬁnally made good on his promise at some point between Yavin and Endor, and that the two studied the ability to defeat death together. In that case, he would have learned the ability as Vader, rather than as Anakin — and in that case, he learned a twisted Dark Side version of the ability.
In either case, Anakin's Force spirit manifests as the young pre-fall Anakin, and not the post-redemption Anakin. It appears that only the part of him that was Anakin lived on in that Force spirit, without any trace of the part that was Vader. The corrupted part of his consciousness that lived during his evil years appears to have been purged from Anakin's spirit.
But what happened with the corrupted part of Anakin's consciousness, the part that was Vader during his evil years? One possibility is that it is simply gone, extirpated during Anakin's redemption. But another option is that it also lived on as a Force spirit, and where Anakin's spirit would be a pure and uncorrupted version of Anakin, Vader's spirit would be all Vader, pure corruption. I submit that it'd be a hideous exaggeration of Vader's corrupted features, and that it would look like this:
Left: Vader, right: Snoke
The giant, translucent, shimmering apparition of Snoke (who we have thus far never seen in the ﬂesh) that talks to Kylo Ren and General Hux is Snoke, not a hologram of him. Snoke himself is the Force spirit of Vader in his most corrupted form, purged of Anakin's good just as Anakin was purged of Vader's evil.