Who, or what, are the Shadow People?

Many people have reported seeing the black outlines of people watching them for brief moments before disappearing. Sometimes the shadows are said to have glowing red eyes. This phenomena seems to be on the rise.

I have heard theories ranging from fairies to time travelers regarding their origins. What do you think? What are these beings? Do they even exist, or are thousands of people simply hallucinating the same thing. If it is just a hallucination, what is the cause of it?