I see them all the time since I lost most on my eyesight. I hear things also. Sometimes my wife calling me, but she hasn't yet. She was going to. I can't explain it and I don't trust my eyes.
Mainstream science says that it is nothing more than the active human imagination. It's our minds playing tricks on us... our eyes seeing things in a fraction of a second that aren't really there - illusions... real shadows caused by passing auto headlights, or some similar explanation. And without a doubt, these explanations probably can account for some if not many experiences. The human eye and mind are easily fooled.
The dark countenance and malevolent feelings that are often reported in association with these creatures has led some researchers to speculate that they may be demonic in nature. If they are demons, we have to wonder what their purpose or intent is in letting themselves be seen in this manner. Is it merely to frighten? They don't seem to be like ghosts which mostly are a different color. I think I don't want anything to do with them. If I leave them alone, they will leave me alone and that is fine with me.