Unknown reason for dark underarm, what should I do to whiten it?

I don't know the reason for my dark underarm. It appears even before I started to shave for the first time or use deodorant. But when I was younger, I sweat a lot(underarm) and perhaps didn't take of my hygiene so it went dark? Because as I look at my dark spot, it doesn't go all even, there's a line that is just normal toned, so I'm guessing it didn't sweat there. So if that is the reason, how can I whiten it? Link for pictures: http://i.imgur.com/g4hwyPw.jpg http://i.imgur.com/Syq58j8.jpg http://i.imgur.com/6b3f88g.jpg