What is this?

This is right under my breast, like on the top of my ribs.

These dark patches of skin will randomly appear on my skin and I’ll scrub them and moisturize but it won’t go away. What is it?

I’ve dealt with it before and it kind of went away on its own but now it’s back and a lot more

EDIT: no itching, not dry either. It just feels like skin but it keeps popping up