24" White iMac Screen Issue

[SuperAcer79]

I have a 24" white iMac and a few days ago I noticed a medium sized gray blotch halfway down on the left side of the screen. It's only visible against a white background. It's anything on the exterior of the screen as I thoroughly cleaned it. What caused this and will it get worse?

 

[SuperAcer79]

Well after a year, the spot has gotten grayer/darker and larger and definitely more noticeable. It doesn't show up if I take a screenshot. I've read that "burn-in" was a problem with iMacs. Is this screen "burn-in" from too much use? How costly would it be to repair this issue and would it be worth it? I really like this screen because I have yet to find another 24" non-widescreen monitor. The biggest size I see for sale is 19".