Shadow People have existed in all cultures since prehistoric times. Many ghosts and demons in folklore share striking similarities to the Shadow People, being tall, shadowy, and vaguely humanoid specters whose presence brings an unshakable feeling of doom, dread, or despair on any unfortunate people who stray in their path. Often, these creatures were seen as harbingers for great destruction or ruin, such as the death of those who encountered them, the failing of harvests, the coming of plague, or even in some cases, the arrival of a natural disaster, such as a flood or earthquake.
In the fields of paranormal study, the Shadow People are discussed in length with varied theories ranging from the spirits of the dead, "echoes" of past or future events, parallel dimensions, and even demonic entities or psychic vampires.
In cryptid studies, the Shadow People are not as explored, but theories range from an undiscovered humanoid or possible alien connection.
Possible Scientific Explainations
The Shadow People are recognized as a common hallucination when people suffer Sleep Paralysis, a condition once blamed on many "nocturnal spirits" such as succubi, night hags and goblins. The condition causes the body to become paralyzed, but also causes the brain to remain active, causing horrifying hallucinations and a sense of dread.
Due to their deep cultural impact dating back to prehistory, the Shadow People have inspired many iconic creatures in popular culture, such as the infamous internet-based horror figure known as Slender Man. They are also frequently found in "creepypasta" material, as well as in media concerning the supernatural.