Hi @bugvi
The TV has edge lit backlighting so it may be a faulty [https://www.google.com/search?client=firefox-b-d&q=UE55NU6025K+backlight+strips|LED backlight strip] (examples only to show cost - make sure you select ''your TV model number'') or a faulty LED backlight driver.
I couldn't find a video online that shows how to replace the backlight strips in your model (you may have better luck - edge lit TVs are slightly different to TVs with "normal" backlighting i.e. strips are behind the screen in horizontal rows and not just on either side of the screen) but here's a [https://www.youtube.com/watch?v=BCu19x3uTOg|video] that may help to test if it is the LED driver that is the problem.