It would seem the display adhesive and/or rubber gasket has lifted or separated slightly. If so, then dirt could more easily get inside the display. I honestly don't know if the glass & panel are glued together of if the panel & glass are separate. If the latter, then any dirt which enters inside could get between the panel & glass obscuring the view a bit.
If you don't have AppleCare+ coverage, then the repair would be expensive. You would have to have Apple or an Apple Authorized Service Provider examine the laptop to provide you with a repair estimate since prices can vary between models & regions.