Weird stain on Mac

hey guys, i was just looking at my macbook screen on a dark web oage and noticed i have this stain in the middle of the screen, it doesnt show when the laptop is off or any page that has is semi bright, but noticed it ion anything thats dark

any idea on what this is? i dont notice it and didnt know it was there until recently, so its not bothering too much

http://imgur.com/trtSDx3

thanks in advance :)