I have dark circles under my eyes that have been her for months. Sort of look like bruises and not from a lack of sleep. Cause & treatment?

Dark circles. The most common causes are allergies and hereditary dark skin pigmentation. Other issues can also cause this problem such as poor health, malnutrition, alcohol abuse, etc. Treatments include cool compresses, allergy meds, alcohol cessation, balanced diet, makeup concealer, dermal filler, laser skin treatments.
Many causes. Dark circles have many causes, and treatment depends on the cause. Those due to sun damage, for example, are treated with commercial lighteners and sunscreen. But if the skin is just thin, allowing the bluish muscle underneath to be visible, there is no treatment other than teamine & makeup. So don't waste $$ on treatments until you see a dermatologist and get a precise diagnosis.