Less contrast in section of screen, around apple logo

Hello,

Since a few weeks i'm having a section of less contrast on my LCD-screen.

You can only see it with a dark background, it is like there is some moisture under my screen, but this should not stay there for a long time if it really was moisture.

It is on the same place where the Apple-logo stands at the back of the screen. Actually around the place where the Apple-logo stands.

Does anybody know what it is ? And if yes, do you know what you can do with it ?

Thank you very much in advance,

Thomas