It sounds like your TV might be having an issue with its backlighting. It could be a problem with the LEDs, but since the screen seems to be evenly lit it's more likely an issue with the settings or circuitry.
It's worth trying a few basic troubleshooting steps before taking it in for repairs. First, try changing the picture mode and check if that helps adjust brightness levels. If that doesn't work, you can also try resetting the TV to factory settings or updating its software.
If none of those solutions work, then it may be time to take your TV in for repairs or contact customer service for further assistance.