It's hard to tell from your photo but it could be backlight bleeding from the edges of the screen. If it's not noticeable in actual use you really don't have to do anything. Almost every screen on an iOS device shows at least some backlight bleeding from the screen edges when you view a black screen in darkness. There's nothing that can be done to correct the problem other than exchanging your device if it's still under warranty. If your warranty is still in effect and Apple agrees that the bleeding is excessive, you may be able to get a free exchange.