This is one of the most often reported and vexing phenomena in paranormal research.
There are a lot of people projecting beliefs onto them because these "beings" are dark and without features they must be demons/evil, etc. That is not research; that is projecting ones beliefs.
Belief shouldn't enter into any of this research. One should let the evidence form the belief, not belief form the evidence.
That being said, observe behaviors. I did this my entire upbringing at Aspen Grove. I studied the patterns, the activity, the actions and reactions. We had nothing to be afraid of in the least physically, but we could be startled or surprised and to some that is bad/evil/menacing.
Sometimes we see a horror movie that makes us jump in surprise a lot and when it's over we say "that was so scary." No, "that was so surprising."
What we know of shadow people is that they are often first seen out of the corner of the eye, not straight on. Likely this is because they are in movement and movement is often discerned by that part of our visual field. By the time we turn our head to look at it, it's either gone or speeding off and disappearing.
They are also often associated with feelings of dread or doom. This makes people feel threatened. This association could be a side effect of however they form or project. Perhaps they are putting off an electrical or other energy that we perceive through our senses.
They seem to think we can't see them and they are shocked when we do and they rush away. This says a lot. It says that perhaps man's ability to perceive them is developing. Some say this might be the result of staring at fast-refreshing screens and lit monitors.
These forms also can show up anywhere, not necessarily haunted locations. They are actually seen outdoors more often than indoors and in very isolated places like parks and cemeteries.
All that we know is thus far, they do not hurt, they are shy and the only real harm is our own natural preservation instincts rising upon witnessing something unexplained.
What are shadow people?
The greatest clues we have about shadow people include:
They can come and go without needing an entrance or exit so they are obviously fluctuating from this visible realm to another or from invisible to visible.
They are usually short and always completely black with zero features. This "black hole" aspect is significant as it shows the lack of features is a sign of incomplete formation or perhaps that these beings literally suck the light out of the space they are taking up, leaving them a space that has no visual content.
With that in mind, we may be seeing only the aspect of them which we can comprehend.
Carl Sagan did a great video on YouTube (above) of what the 4th dimension is like. He said, if you put your finger on a 2-dimensional photograph, the 2-dimensional being would see only the 2 dimensions of you which would appear to them like one of their own, suddenly appearing (and when you lift your finger away) suddenly disappearing like magic.
So I think it is likely with the shadow people, the only part of them we can see is the 3-dimensional aspect. That they may actually take up more space in another dimensional way might explain why we get such a negative feeling from them, depending on how their other dimensional aspects cross our 3-dimensional bodies.
Think of it like this - that finger on the photograph looked like one of their own in 2 dimensions, but it leaves heat. That's not a visual aspect, but a feeling....
I always advocate studying actions and reactions when it comes to the unexplained. Assess threat versus your belief system or the way your mind interprets what it sees as a threat.
We continue to study these odd beings as we have another similar phenomena called "black-eyed kids" (BEKs).
In this case, they try to ask for assistance and gain access to cars and homes. They look like children, but give off a very doom-filled odd sensation that makes ones self-preservation instincts rise up. Their eyes are reported as having no whites, only black.
If you were a being that could not easily get around this realm without being detected, and were surprised that 3-dimensional humans could perceive you, would you not try to hide as one of us? Being short, what is the likely candidate to imitate?
It's only a vague consideration of mine, but one I hope to tie together through as many witness encounters as possible with both beings.
Let's continue to ask questions of our magnificent universe. It does not just contain this world, but others....