Mark keeps reappearing on iPhone screen?

I've had my iPhone 4 for less than a month, and since like the first week of having it there's been a mark on the screen which, no matter how many times I wipe it off, keeps coming back. Is it because of a crack on the screen or something else, and how could I possibly fix it?

It is a slight crack. If you dropped your phone it could appear. If you bought a referbished phone (used) the crack must have came from the previous owner

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