One interesting idea suggests that shadow people are the shadows or essences of people who are having out-of-body experiences. According to Jerry Gross, an author, lecturer and teacher about astral travel, we all travel out of the body when we are asleep. Perhaps, this theory says, we are seeing the ephemeral astral bodies of these twilight travelers.
People from our own future, another idea states, could have found the means to travel to the past - our time. However they are able to accomplish this incredible feat, perhaps in that state they appear to us merely as passing shadows as they observe the events of our timeline.
Even mainstream science is fairly convinced that there are dimensions other than the three we inhabit. And if these other dimensions exist, who or what (if anything) inhabits them? Some theorists say that these dimensions exist parallel and very close to our own, although invisible to us. And if there are inhabitants in these other dimensions, it is possible that they have found a way to intrude on our dimension and become, at least partially, visible? If so, they could very well appear as shadows. It has long been held by psychics and other sensitives that beings on other planes of existence are of different "vibrations." Science is beginning to look at reality, on a quantum level, in the same way - that particles of the smallest size exist as vibrations. Perhaps, some theorize, the vibrations of our existence are beginning to mesh with those of another dimension, which accounts for the increase in such phenomena as ghosts, shadow people and possibly aliens.
The alien and abduction phenomena are so bizarre that it's no surprise that extraterrestrials are suspects as the shadow people. Abductees have reported in many cases that the alien grays seem to be able to pass through walls and closed windows, and to appear and disappear abruptly, among other otherworldly talents. Perhaps, too, they can go about their alien agenda disguised in the shadows.
There's a good deal of overlapping among the above ideas, of course. Aliens and ghosts could be interdimensional beings, or aliens could be time travelers - and some believe demons are responsible for all of these disturbing phenomena.
There is no way to prove or disprove any theories about a phenomenon that is so mysterious, that happens so quickly and without warning. Science finds it virtually impossible to catalog or study such phenomena in any methodical way. All we can do, at present, is to document personal experiences and try to piece together what the shadow people phenomenon might be. Perhaps it's an old mystery becoming more recognizable... perhaps it represents a doorway to and from different planes of existence... or perhaps it's just shadows.