If its a LCD tv then the backlight inverter needs replaced if it is a LED tv then you have a Bad led or LED’s and they need replaced You would have to take everything off the tv top get to the LED’s. It’s not that hard but it is really time consuming and a tv guy will charge you $200-$300. but if you do it you could buy the leds of ebay and led tester off ebay also might coast $50-$100. Or if you lived close i would do it for $50 :)
I had that problem witrh my Samsung and replaced the main ogic board and it worked. Ithink it could be any one of the 3 boards tho. You will have to check the voltages with your multi meter. Chek out youtube. Tampatec has a good video on checking voltages.
more than likely the LCD panel is going bad. I came across one tv the a ribbon cable wasn't in all the way but there was a dark part of the tv about inches going top to bottom. Pinch the bezel a few times around where the dark spots are on the top or bottom. if they move then you have a loose connection..if they don't move and they are getting bigger it's a bad panel.