There's many different things that can cause dark circles under the eyes, some avoidable - others, not so much...
Genetics - unfortunately, dark circles can be hereditary.
Thinning skin - as we age, skin thins and the area under your eyes can be greatly affected because the skin here is already thin to begin with. When this area thins with age, blood vessels become more visible and therefore, darker.
Lack of sleep - if only I could sleep all day! :)
Anemia - an iron deficiency. Skin looks more pale and area around the eyes look darker.
Lack of nutrients - an unhealthy diet can affect your skin in SO many ways, dark circles included. Try a more balanced diet and your skin will love you for it!
Dehydration - drinking water goes hand in hand with a balanced diet. Drinking enough water daily will help your body flush toxins.
Hormonal changes - gotta love being a woman! Pregnancy, menopause and menstruation can cause dark circles and puffiness.
Allergies/Hay Fever... Smoking and drinking also does not help! :)
How to prevent them:
Drink lots of water throughout the day.
Try your best to get a minimum of 7 hours of sleep per night. Also, you can sleep with your head elevated (add an extra pillow) - this will help prevent any blood from pooling under your eyes while laying down.
Always wear SPF! (sun damage can darken your circles)
Slices of potato or cucumber - place on closed eyes and keep them on for around 15 minutes.
Cold compresses can help with both dark circles and bags.
The caffeine in tea bags can help diminish both dark circles and puffiness. Leave on your eyes for 10-15 minutes (make sure it's not too hot!)
Take your daily vitamins and eat as healthy as possible.
Eye creams - collagen, vitamin C, retinol. Gently pat in below and around eyes with ring fingers.
An immediate relief from dark circles? Concealer! Pick a shade the same color as your skin tone or a shade or two lighter. Pat in with ring fingers then blend. Set with a powder (Ben Nye's Banana powder is a huge favorite of so many).
Any questions or requests, feel free to email me! firstname.lastname@example.org