Why does the pupil dilate when its dark?

The pupil of your eye controls the level of light entering your retina were the little micro sensors of your eyes are stored. When it is dark, you pupil dilates, letting in more light, even though there isn't much, therefore making it easier to see in the dark.

Extra details: In the dark, you also have specialized parts of your eyes, that take a while to become active, that help you see in the dark. These parts stay inactive when it's bright because the light burns them. That is why it hurts your eyes to go from dark, to bright in a quick flash. Also since they take a while to become active, there also the reason it takes a while to be able to see in the dark