On one of my eyes i've noticed i've got a very dark under eye circle and on the other eye it's not as dark, why is this?

Probably same : Circles under eyes, if a systemic, are mostly bilateral and uniform. Many causes are identified, including nutritional deficiencies or anaemia, Harmonal imbalance or even lack of sleep. You may notice a difference in size initially but that could be skin shade effect or matter of development. You can always develop a unilateral local skin condition but usually will have different appearance.