Not clear to the understanding; not easily seen through; obscure; mysterious; hidden.

Absence of light; darkness; obscurity; a place where there is little or no light.

To darken; to obscure.

secret; "keep it dark"; "the dark mysteries of Africa and the fabled wonders of the East"

marked by difficulty of style or expression; "much that was dark is now quite clear to me"; "those who do not appreciate Kafka's work say his style is obscure"

Anything secret is said to be dark. "Here's the dark of it," is a way of saying "I've got a secret and I'll share it with you."