It is an unfortunate reality that LCD screens (typically ones using fluorescent backlighting) can suffer from "clouding." This is a phenomenon where the lighting "leaks" to the visible portion of the screen, causing uneven blacks. This manifests itself as areas that appear more grey than black in darkly lit scenes, along with uneven brightness in normal pictures. This is sometimes remedied by a little pressure, and by adjusting the settings on the set.
Access the TV's menu using the remote control. Navigate to the menu areas that adjust backlight and screen brightness.
Lower the backlight to the lowest tolerable level. Often, backlighting can be turned off (or very close to it) without appreciable degredation of picture quality. This will be an exercise in getting used to lowered brightness versus cloudy visibility.
Turn off any automatic settings that adjust brightness based on room light. Often called an "iris" or "smart sensor," these disable the degree of control required to mitigate the visibility of clouds under all viewing conditions.
Turn the TV off. Take a clean, lint-free (preferably brand-new) cloth and wipe, using firm yet cautious pressure, from the centre of the screen to the edges. Repeat until the visibility of the clouds is reduced.
Loosen (but do not remove) the screws along the upper rear of the set. Many times, these screws are so tight that they cause the screen to slightly warp, causing visible unevenness in the backlighting. Remember to leave the screws tight enough, however, to prevent the set from falling apart. Usually, 1/4 turn is sufficient.
Lightly twist the cabinet of the LCD while the screws are loose. Turn on the set to determine if the clouds have been reduced. If not, tighten the screws back up on the rear of the set close to their previous tightness.
Never apply pressure to an LCD screen with the power on.