2. Thinning skin.
If the skin underneath your eye is relatively thin compared to the average person, the dark circles under eye circles could be your blood vessels showing through your skin.
This is due to the violet wavelengths that pass through your periorbital area underneath your eye.
As you become older the pigment and elasticity of your skin naturally begins to thin, so this is another condition that shouldn't concern you because its only effects are aesthetic, and can be easily removed with a few simple treatments.