Hello I Have Dry Dark Brown Patches Around My Armpit.

Hello i have dry dark brown patches around my armpit. They don't hurt or really bother me until it comes to wearing anything that shows my armpits . I've tried keeping them moisturized with baby lotion right after i got out of the shower but it doesn't seem to work. Do you know whats causing is? Or what it is? And can you PLEASE tell me how to get rid of it?