It only shows up in dark images, anything bright and colourful and you can't see this big smudge-like thing. I haven't dropped it or knocked it or anything and I've only had it for 4 months from a Stormfront store in the UK. Very dissapointing 🙁
No, wiping wouldn't have caused this, the glass faceplate is pretty resilient against pressure and the metal enclosure behind the screen is pretty strong against pressure. That looks like a hardware issue. You should definitely take it to Apple asap.
Looks like Mura, also known as bad spots, to me. This should be covered under warranty/AppleCare, especially if there is no visible dents on the display clamshell.