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It looks like you are getting some kind of delamination in your screen assembly. This will require that you replace the screen. Is this the original screen or a replacement?
 
Hey Minho and mattguy10, thank you for your support,
and no, there's not a screen protector and this is the original screen.
I don't know how to fix, I would appreciate your help.
Also, I think this problem started when I tried iPhone 6s case, I'm not sure but I think the case caused the problem.
 
You can repair it yourself if your good at DIY repairs, just follow the guides at iFixit. But if you're unsure, then let a pro do it; either the Apple store or an independent repair shop.
 
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