DIAGNOSIS That's called 'backlight clouding' or 'DSE' (Dirty Screen Effect). It's a physical characteristic of the panel and backlight assembly, not a software bug. On budget-friendly LED TVs like this model, some degree of this is unfortunately common.
FIX There's no software fix to eliminate it, but you can minimize its appearance. Go to Settings > Picture > Advanced Settings. Turn down the 'Backlight' setting (not brightness) for nighttime viewing. Also, try the 'Local Contrast' setting—sometimes setting it to 'High' can help even out the lighting, though it might crush shadow detail.
IF THAT DIDN'T WORK If the patches are severe and look more like actual dark spots or blobs, you might have a defective panel or loose connections inside. This is a warranty claim. Take a photo of a solid gray test screen (search 'DSE test' on YouTube) to show support.
HEADS UP This is most noticeable in a dark room on a black or dark gray screen. Some clouding is considered within manufacturing tolerance, so a warranty replacement isn't guaranteed unless it's really extreme.