Why/how does someone develop accessory axillary breast tissue and what helps make it less visible without surgery? and can cancer develop there

You're both with it: It develops along with the normal breasts. There may be must a nipple, or the deep structures too. You won't make it go away but surgical removal is routine and you'll probably be glad. And yes, breast cancer occasionally arises here. Remember your right to full informed consent from your phyiscian -- you'd do well to get a surgical consult now to learn all about your choices.

Variation of Normal: All women have breast tissue that extends into the armpit; some simply have more. This commonly becomes prominent after puberty or childbirth. While there is no need to remove it, the only option is surgical. Like any other breast tissue, cancer can develop there. Hope this helps!