Hello, few days ago I bought a used Mid 2015 MBP 15 inch. Unfortunately I didn't notice the screen has a mark in the middle. (as I mostly tested it on external screen, because that's how I mostly gonna use it) Best seen when the background is white. Can someone tell what is this? and what may cause it? Or maybe it's possible to get rid of this mark? At least cleaning with wet cloth didn't help Thanx in advance.