I am working on designing the UI of a video monitoring system. After I did some research, I found out that most designers use a dark background to be the primary color.
Does anyone know the reason why?
A very good question and I would say that you are very keen about all the small details used in design.
Here is the best example I think I can give, Take the Sky as background and Moon and stars as video, Moon is sometimes visible in day but stars are not.
Even though the moon is visible, it's not so pleasant as much as it's in night.
I would say a video for me is more like a dance of colors. There are multiple or couple color at a time, which gets changed in even frame.
Human eye is more attracted to dancing colors when they are hosted black background.
Having other color in background might distract them from actual content, which leads to missing of information some times.
Two reasons. First, Keeping the background dark allows you to focus on what's important--the video. Second, a bright background causes your eye to close down making other elements in view appear darker, thus making the video more difficult to watch and see the details; particularly in dark areas.