This description sounds like acanthosis nigricans. This is a condition where your skin​ folds like underarms, inner thighs or groin, neck and face becomes dark. It is either genetic or due to sedantry lifestyle and overweight. This can be treated. Please contact me with photos using online consult option for proper diagnosis, treatment advice and prescription.

You are suffering from acanthosis nigricans causing darkness. Medicine available for good improvement......As the treatment varies depending on the severity... Send photos of the affected area, by direct online consultation for accurate diagnosis and treatment.

Underarm darkening occurs due to many reasons like friction due to shaving, use of certain deodorants, hormonal problems like thyroid imbalance, sudden weight gain, etc. For a more accurate diagnosis, I require some more details from you and photographs of the affected area. Kindly share the same via online consultation on Practo or visit a certified dermatologist near you.