Discoloring/dust at the back of my iphone

So i have bought a brand new iPhone XR white , after only 1 day i have noticed at the back between the glass and the aluminium a discolorisation and dirt inside. I have kept the iPhone in an official Apple clean case and when i took it off i noticed the problem.

I am also very careful with the Phone, my friend has the same one for 6 months without a case and she does not have this problem. I am a little bit sad because i paid lots money for it.

(Edit) I noticed that everyday the dirt and discolourisation getting worse

I still have warranty

So what is a solution?