Huge orange stain on my monitor

Hi,

for a while now i have kind of noticed my screen being too orange, and yesterday after pulling up a full screen white image, i found out the the whole middle of my screen has turned orange while the outskirts are still white as they should be.

i will attach an image so you can see. my screen model is the U2410 and have had it for about 4 years. i checked and it is not under warranty anymore as the date on the plastic pull out slip is Feb 2012.

does anyone know what this issue is and what may have caused it? and if it is fixable? (im guessing not)

i dont think it is burnt pixels. could it be the exterior of the screen itself, the plastic, that has gone bad?

will appreciate any insight, thanks!