The dark opening in the iris of your eye is called the pupil.
The iris is the colored part of your eye, and the pupil is the black circle in the center that allows light to enter the eye. The size of the pupil changes depending on the amount of light available. In bright light, the pupil constricts (gets smaller) to limit the amount of light entering the eye. In dark light, the pupil dilates (gets bigger) to allow more light to enter the eye and improve vision.