I am a redhead and I have a dark brown spot on my breast. What could this be?

I am aredhead, therefore, my nipples are pink in color and my one small mole is pink as well, however, I have a flat, dark brown spot on my right breast. It has been there since I was a young girl. It got to be about the size of a pencil eraser years later, however it has'nt gotten any bigger since I was in my twenties, which has been about 12 years ago. What could this be? It is not painful and it does not bother me, I am just curious to know whether it is something that I should be concerned about or not. I have been told by other women that it may be melanoma. Could that be dangerous to my health?