Theories

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The Hyper-Dimension Theory
This theory suggests that Slender Man (and for that matter, The Virus), are some form of 4th-Dimensional or Hyper-Dimensional beings which protrude into our own spatial dimensions.

This theory does help explain many of the abilities of the Slender Man - Slendersickness is very physically based and could be caused by protrusions within the body for example.

This is mainly backed up by the ability of The Collective to perceive multiple points in time as would be possible for any being outside of normal space - Collective members simply protrude into the body of a host from a higher dimension.

Motives
Based on this theory, there are various potential reasons why Slender Man acts in the malevolent manner it does:
 * It requires the creation of new Collective members to survive
 * It needs to protrude into our dimension to survive (Technically could be true of any 4-dimensional being in order to allow systems to function), and uses Collective members to protect itself
 * It enjoys torturing people

The Tulpa Effect
The Tulpa effect is a theory originating from the series of the same name, which explains Slenderman as a type of thoughtform which became a cognitohazard. According to this theory, Slender Man exists in-universe due to people's belief in him. Basically, humanity has willed him into existence subconsciously. This also could explain the mental trauma that Slender Man is capable of inflicting on people seemingly at random, as an entity of the mind could still interact with people's mental states.

Core Theory
Core theory is a sub-theory that is reliant on the existence of the Tulpa effect. Essentially, you can have the Tulpa effect without Core theory but you cannot have Core theory without the Tulpa effect. This sub-theory was created by Robert of the blog White Elephants, and essentially states that, given Slender Man's status as a thoughtform, he can be defeated or possibly even killed by shifting the way the characters think of him, which Robert states is best done by framing it as a story with various heroes filling different roles. Very meta indeed.

Core Theory Roles
According to the rules of Core theory, the following roles must be filled. Additional roles have been outlined by Bluelight of Speaking of the Truth, but these may not be canon. The 'canon' roles are as follows:
 * Sage
 * Mystic
 * Hermit
 * Warrior
 * Guardian
 * Hero

Time-warping Theory
This theory (created by one Steven Savoy) states that Slender Man is able to "warp" time in order to teleport, slowing time down for everyone but himself so he appears to move from place to place without being seen in any of the areas in-between. This theory also goes forth to state that Slender Man can slow down time for everyone including himself, but is affected to a greater degree than that of his victims, which leads to the often slowed-down movements seen when he isn't agitated.

Iteration or Cycle Theory
This theory pertains to EverymanHYBRID in particular. It relates to the idea that Vinnie, Evan, Jeff, Stephanie, and perhaps Dr. Corenthal, Slender Man, HABIT, and the Rake, and others are reincarnations of past versions of themselves, locked in a sort of time loop lasting at least several decades, in which they fight each other every so often.

The theory suggests that the characters "respawn" when they die until the "game" is over, when they are all sent to another realm known as the Candleverse. It also suggests that to win the "game," (read: survive) the protagonists must either break the cycle or kill all three monsters.

The biggest pieces of evidence for this theory are the Fairmount versions of each of the four protagonists, the existence of Princeton!Vinnie, and the leaked video "Welcome to The Ark," which may or may not be canon. Smaller pieces of evidence exist as well, but the theory is all but confirmed by what we see throughout the narrative.

Death Iteration Theory
This sub-theory states that, in addition to the periodic large iterations, the cast also experiences small iterations whenever they die. This sub-theory suggests that the cast have died many times both on- and off-camera.

The Candleverse
The Candleverse, also nicknamed "Candle Cove" and known as "The Ark," is the speculated location of the characters' non-earthly bodies between cycles. This speculative realm is seen in many of the "hidden" videos, and is also thought to be the location of Corenthal's "Eden." The whole realm, with the exception of the aforementioned "Eden," is submerged in a perpetual darkness, and thus candles are a precious amenity. This is where the name comes from. The moniker "Candle Cove" comes from conflation of the Candleverse with the Creepypasta of the same name, although based on snippets of conversation between characters and other hints throughout the series, "The Ark" may just be the most accurate name for this realm.

It is believed by some that the final video, which takes place on Baldpate Mountain, actually is in the Candleverse, and that the cycle has finally broken, leading to Corenthal's "Eden" becoming the entire Candleverse.

Quantum Theory
Quantum theory is the idea that all of Slender Man's abilities and actions can be explained using applied physics. Basically, the action of "Slenderwalking" or "teleportation" on Slender Man's behalf is due to Slender Man acting as a waveform, which only locks in place when being observed, i.e. he both exists and does not exist in any given location until that location is observed, in much the same way as Schrödinger's cat is both alive and dead at the same time until the box is opened and the cat assumes one of the two states.

SCP Theory
The SCP theory suggests that Slender Man is an object from the SCP Foundation universe, specifically SCP-582 or the "Bundle of Stories."

This seems to be the acting theory in the quasi-Slenderseries Monarch's 52, which deals explicitly with the SCP universe, albeit its own version.

Species Theory
The Species theory suggests that Slender Man is a species of animals roaming forests. Futher proven by the appearance and presence of The Director, The Virus, and the other Nightmares in Recollect.

Although this theory does not apply to The Operator, it could be applied to other incarnations of Slender Man, such as Gorr'Rylaehotep and The Administrator.