Dark butt crack

Why is my buttcrack dark only where my cheeks touch? I have a large mirror in my bathroom, and today’s the first day off my period. I was cleaning myself up over the toilet and noticed when I spreed my buttcheeks it was unusually darker than the rest of my body. It can only be seen when I spread my cheeks wide open. I have a huge butt, but I’m petite. I don’t wear thongs or hipster cut panties, but I’m considering only wearing them now because I wear tight granny panties and I’m thinking that this is from friction from tight pants and panties. I can’t get any decent answers on google without anyone responding sarcastically. I’m genuinely concerned. I’ve never noticed it before but I’m sure it wasn’t always like this because I would’ve noticed. It’s not really an issue but it is a little bit embarrassing and I want to know how to get rid of it. I haven’t dated or slept with anyone in almost two years, and before that I’ve been with the same guy since I was 16 but we mostly had sex in the dark so I don’t think he ever noticed it. Has anyone else had to deal with this? If so, how do you get rid of it? 😰