Although @oldturkey03 is our resident flatscreen TV expert, I'll throw in my $0.02 worth.
If I had to guess, I'd say those are dirt streaks, probably from condensation at some point that got in between the panels of the display. There are actually multiple layers, made of up diffusers and polarizers and the LCD itself. It's entirely possible that you could disassemble the screen and clean off the dirty layer. Once you reassembled everything, it's quite likely you could have a screen that looks new again.
I didn't locate any information on your TV specifically regarding how to do that teardown, but I did come across the service manual, which unfortunately stops before it shows how to take apart the panel.
Samsung UN43TU8000FXZA Chassis UWK00, Service Manual, Repair Schematics
Best best would be to search out videos showing how to replace the backlight LEDs since you have to take apart the panel to do that.