Our brain is capable of naturally producing hallucinogenic compounds,
notably DMT, what the point of this compound is in our brain is unclear - maybe our brain simply has an, "I'm bored, let's trip" button somewhere. Maybe it's meant to ease our fears of death - and provide "afterlife / white light" experiences.
Research into DMT experiences provide an oddly consistent account of humanoid forms, "Machine-elves", "Dwarves", "Aliens" - perhaps also Ghosts. Conceivably, much of our folklore, or religious visionary experiences may also be the result of endogenous DMT.
The best explanation for virtually all paranormal activity, to my mind, is that our brain decided to inject us with an extremely potent hallucinogenic. It may also be the case that there is some kind of trigger for when DMT is to be released in our brain - in which case shared experiences may also be the result of simultaneous triggering (ex. some kind of rare spore exposure, or sleep deprivation, or fear, etc).