Dark hair on breasts or the general area of the chest is caused by high levels of androgens, the so-called male hormones. Androgens include testosterone and cause the appearance of many secondary sexual traits at puberty, including body hair growth. Individuals are affected by androgens regardless of gender.
Testosterone is the androgen responsible for patterns of hair growth and loss on the human body. Parts of the body, such as the eyelashes and eyebrows, are less sensitive but are still impacted by testosterone levels. The pubic area, face, underarms and chest are more sensitive to testosterone levels and develop hair following the surge of sex hormone production at puberty.
Dark hairs found on the scalp or pubic area are called "terminal hairs." The lighter, finer hairs that cover most of the body are named "vellus hairs," and are converted to terminal hairs by the action of testosterone.