I've had experiences with what are commonly called 'shadow people'. ( well thats what I think they are ) Rather unpleasant experiences at that. There was a thread about it somewhere on the forum and if I cant remember correctly, the person who started it was skeptical as to whether they actually existed. They do exist, I can tell you that much. I'd see them often...I'm not sure why I did? The encounters got worse as I got older. At first I'd just see them standing in hall ways, door ways and darting across rooms. Then they started tampering with things. Moving stuff, slamming doors, switching off lights and appliances around the house.
One night I was asleep on my bed when I saw this thing staring at me from the door way. I stared back for a while, I started to get frightened, started breathing heavier, I froze. It darted over and slammed my body to the bed. I actually felt my body being pushed further into the springs of the mattress, and thats where I began to stop breathing. I was so frightened I could only inhale - Literally gasping for air. Then it left??
The experience is still overwhelming, still bothers me.

I told a close friend about what had happened, how it made me feel. Her and her mother went out and bought me Quan yin ( the female Buddah ) and since then I haven't experienced this entity. Bot even a glimpse.
She has had a massive impact on the situation and for that I'm thankful but I'm still curious as to why I was seeing those things in the first place?

Sometimes Id see darkness expanding from corners of the room around the same time I'd see these things. Is it just a mass of negative energy? And if so...is it mine or something else?

I do not what they are but I have seen them too. Not as often as you but twice. The first time I saw it was eeriely similar to one of your encounters. I saw it stand in my bedroom doorway and I stared at it for a few moments. At that age though I was extremely frightened and just shut my eyes extremely tight and didn't look again. Then a few years latter, last year actually, I saw one standing inside my room. At first I thought someone was in my room and asked who it was. It just, I don't want to say exploded but it broke into hundreds of small fly size pieces and leeched into the surrounding darkness.

I do remember the encompassing darkness in their presence.

I'm in the dark right now, and I can tell you, this won't be fun to write about.

I was 8, and I looked out of my window and saw a group of shadows walk across the window. They were demon-like, and I remember that one could fly.

Earlier than that, I was laying in bed and staring at my ceiling, and shadows began to engolf the corner of my room. I screamed, and next thing I knew, was on the floor.

More recently, I'm able to see things. Flickers in the dark, movement, and I can feel something very strongly. I'm freaked out right now, and I don't know if what I see or feel is "astral bodies" or what... input would be a blessing.