Dark circles under the eyes are an extremely common and distressing issue.
Dark under-eye circles begin in the capillaries, tiny blood vessels that web the delicate skin around the eyes. Capillaries are so small that red blood cells sometimes have to line up, single file, to get through. Frequently, red blood cells get lost and wander into the surrounding skin. This isn't a problem - it happens all the time - and your body has a mechanism to mop up these escapees. Enzymes in your body break down the red blood cells, including their hemoglobin (the molecule that gives them their distinctive red color).