Dark circles under the eyes can be caused by several things. The most common reason is that the skin becomes too thin in the eye area. It often happens when you get older, but even if the skin becomes dry and lose moisture and volume. When skin becomes thinner the underlying blood vessels becomes more visible and thus arises the dark circles.
Stress and too little sleep can also make you get an altered blood circulation in the skin. This also results in dark circles.
To combat dark circles under the eyes, it can be a good idea to review your sleep habits, you may start exercising and try to reduce the daily stress.
A quicker solution is to use an eye cream that moisturises the delicate skin under the eyes and hides the circles thanks to covering pigments.