What are these dark marks under my arms?

O MY GOSH! IHAVE THAT TOO! AND I HATE IT! I ALWAYS WEAR LIKE LONG SLEEVED SHIRTS OR AT LEAST NO TANK TOPS BECAUSE THIS ONE TIME WHEN I WAS A KID A FRIEND OF MINE ASKED WHY I HAD DARK PUPLE LIKE ARMPITS! I WAS HORRIFIED BBECAUSE I THOUGHT IT WAS JUST A NORMAL THING, I DON’T KNOW WHAT IT IS… IM 16 BY THE WAY.

I just turned 17, and I am female. I can’t really remember when it started to show up, but I don’t remember having them before I was 13 or 14…which was around the time I started wearing black (If that even has anything to do with it.)

I thought maybe the dye had something to do with the marks, because I once bought a robe and I wore it after I took a shower, and since it was new, the water on my hair made the dye bleed onto my back..so I was thinking maybe the dye bleeds into my pores when I sweat? And that the dryness might be from my deoderant or something. I’m just not sure if anyone else had heard of this problem or knew how to take care of it.

Thanks for your answers, by the way. They are very helpful. I’m trying to look into this as much as I can.

Im the same im sooo paranoid about it, Im going to give bio-oil a go. maybe you should to as its for un even skin tones. I wear a lot of black too I personally think its the deodrant ( I’ve tried loads of diffrent types )… which make no diffrence at all. If anyone has managed to get rid please please let me no!

Thanks