It could be a screen burn out issue, that can happen with phones over time, just because of how the display works. How long have you had it? If it's been less then a year, you might be able to get it replaced under watery.
At this point, I would recommend having a certified Samsung technician evaluate the device. You may have two options for this depending on your location. You can either visit a local uBreakiFix location or you can ship your phone into the Samsung service center. If you do not have a uBreakiFix location near you or you would rather ship your phone into the service center I'd be more than happy to get that set up for you I'll just need you to send me a private message with the full model code and IMEI number of your device.