I can't be sure without seeing you, of course, but I think you may have acanthosis nigricans. (This literally means "dark thickening.") It's pretty common and most of the time of no significance beyond appearance. I recommend that you consult a dermatologist for diagnosis and perhaps treatment with topical creams which can help smooth and lighten the skin. It's slow going, but improvement is possible.
Take care.
Dr. Rockoff