What Causes Dark, Rough Patches On Buttocks?

I have a dark, rough skin patches on each of my butt cheeks. It s been there for as long as I can remember. I tried to exfoliate really hard and moisturize, but it doesn t come off. It s where I sit. I am very skinny so when I sit, I sometimes feel my bones pushing on the chair because I don t have enough fat in my buttocks so maybe that might have caused it? How can I get rid of it?