Allergic shiner: This condition is caused by allergies, which can lead to increased blood flow and swelling under the eyes, resulting in a darkened appearance. It's a common and relatively harmless condition.

Fatigue or sleep deprivation: Lack of sleep or fatigue can cause palpebral bags or dark circles under the eyes due to increased blood vessels and fluid accumulation.
Dehydration: Inadequate hydration can lead to a decrease in skin elasticity, making the under-eye area appear darker.
Aging: As we age, the skin under our eyes becomes thinner, and fat pads can shift, causing a darkened appearance.
Trauma: A blow to the face can cause bruising under one eye, leading to darkening.

Basal cell carcinoma: Although rare, basal cell carcinoma can present as a darkened area under the eye. Early detection is crucial for effective treatment.
Orbital cellulitis: This is a serious infection of the tissues surrounding the eye, which can cause swelling, redness, and darkening under the eye. Prompt medical attention is necessary to prevent serious complications.
Thyroid eye disease: This condition can cause swelling and darkening of the skin under the eyes due to inflammation and fat accumulation.

Melioidosis: A rare bacterial infection that can cause skin lesions, including darkening under the eyes.
Sarcoidosis: An autoimmune disease that can cause inflammation and darkening of the skin under the eyes.
Amyloidosis: A condition characterized by the accumulation of abnormal proteins, which can cause skin discoloration, including darkening under the eyes.