|... it might be worth entering your bios and staying in there for a while just to see if there is any difference to the screen growing darker|
... <Fn> + <another key> on the top row of your keyboard will act as Function keys together to brighten/darken your screen a different key will serve another purpose and they should have some sort of icon on the keys!
... lets hope thats all it is!
... Posting is provided "AS IS" with no warranties