Answer:
I am a portal to another world which you cannot enter. Only you can see me but I can't see you.
I fill a room but take no space.
What has no beginning, end, or middle?
Although glory but not at my best. Power will fall to me when the man who made me dies.
What goes up and down, but does not move?
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.