It's possible, though unlikely, that a smudge or spot on the glass panel on your iMac's display is actually on the inside surface.
O kwere omume, ọ bụ ezie na o yighị, na ọkụ ma ọ bụ ntụpọ n'elu panel ahụ dị na ngosipụta nke iMac bụ n'ezie n'ime elu.