The first thing to know is that it is perfectly normal to have a dark discoloration of the skin on some areas, such as the underarms, hands, face, knees, elbows, inner thighs, bikini line, and the buttocks.
These dark areas are an accumulation of dead skin cells and melanin on the upper layers of the skin. Nobody in the world has a pigmentation of the skin perfectly uniform. Some people are simply born with this type of pigmentation, often in intimate areas.
The skin of these areas may also be darker due to other factors, such as pregnancy, hormonal imbalance, or simply aging.