Greetings. This may not be an issue with cleaning of the skin, rather it may be acanthosis nigricans (AN) - which is a hormonally mediated pigmentation . you could send in pictures of your condition / see a qualified dermatologist near you - for confirmation of diagnosis. AN is common in over weight people / diabetics and may be a sign of insulin resistance in family and occurence of diabetes in future. So you must lose weight. Warm regards Dr Govind S Mittal, Excel care hospital, Bengaluru