Big boobs and clievage?

1 of my older cousins has really big titts no homo and ido to but one day she was wearin a low cut shirt I noticed her clievage between her boobs had dots and lookd like the back of a neck I asked her what it was and she said she didn know.. When I was younger I had chicken pocs real bad on my chest by then my **** grew in 38ddd and thier not really noticabl but I wana know what this is so it doesent happen to mine . I know some times they get sweaty so I put baby powder and my big cuzin alisha wants to know some good ways to get rid of a black neck and inbetween her boobs. Its not that shes dierty she never stinks and she showers night and morning