You are likely to be suffering from acanthosis nigricans. It is a condition where your skin becomes thick and is not "dirt". It does not come off by scrubbing. Acanthosis nigricans occurs when you gain a lot of weight or if you have a family history of
diabetes. It is related to a condition called insulin resistance. You will need to use creams which exfoliate your skin and lose weight. Visit a dermatologist near you for treatment of this condition. Warm regards. Dr. Rajdeep Mysore. Chirag Hospital Bangalore