Shadow People are on of my all-time favorite subjects of unexplained phenomena. It seems to be the least researched and yet very widely seen.
Described as anywhere from the usual 3-4 feet tall to upwards of 6 feet tall, solid black looking, shaped like a human, seen out of the corner of your eye first, and then when they realize they are seen, they vanish. They can occur in parks, new homes, office places, scary spaces, haunted locations, and cemeteries. It doesn't seem to matter where. They appear to think they are alone and seem a bit astonished to discover we can see them, as if they were always undetected previously.
So, has something changed in modern man?
One theory is that because of fast refreshing screens on computers, man has become better at capturing things in the visual spectrum than he once was. I'm not sure about this. My vision seems beat to hell from lit screens. It's an interesting concept though. Many people after long days at the computer report seeing them, but that could be incidental since almost all of us use computer screens extensively.
I saw a shadow person one time that actually was able to turn on a computer and make a receipt machine turn on and spit out receipts. So, they do seem to be able to affect the environment. One witness reported putting a camera's target light on one to find it was solid--the light sat on its dark dense body and not through to the wall behind it.
There are theories also including that they are demons, elemental beings like elves or fairies, and that they are a ghost trying to form.
Personally, I'd be most likely to say they are interdimensional travelers. What we are able to see of them is only a portion of their entire self. We can only see 3 dimensions of them. Imagine if we were staring at a 2-dimensional picture of ourselves and then the picture suddenly spoke to us! That's kind of what they might be experiencing. They are messing around with 3-dimensional environments and suddenly those people in 3D can see them and they are shocked.
We really don't know yet what they are and why they are. I think it's telling that we first see them out of the corner of our eye. It's as if that peripheral vision uses a different aspect of our focus. I hope to collect more shadow person stories and learn about commonalities.
For shadow people encounter stories by dozens of people who have encountered them, go to this good archive site.