Dark dry flaky skin right below my underarms?

So I have dry dark flaky skin just under my armpits. I've had this problem for awhile now but I was not born with it, It just appeared out of nowhere! But I noticed my right side is significantly more obvious patch then my left side, left side is actually almost completely gone, just a small spot there. I also... show more So I have dry dark flaky skin just under my armpits. I've had this problem for awhile now but I was not born with it, It just appeared out of nowhere! But I noticed my right side is significantly more obvious patch then my left side, left side is actually almost completely gone, just a small spot there. I also have this problem on my neck. I know its not dirt because I tried scrubbing there so many times but wont go away. What could it be? Do I just need to moisturize more? Any help will be grateful! :)
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