When I was younger (from around 7-12) I would see them, they were always all black. They looked like 3D shadows. Sometimes I would look at them and they would disappear immediately. One that I've looked at ran right in front of me and disappeared into my wall. When I was outside with my friend, we both saw one. I've heard a lot of different theories on them; that they're aliens, demons, ghosts, etc. Personally, I think that they are spirits from another dimension. I wonder why I don't see them anymore. But I am curious to hear about what other people think.