Some causes:
- Fungal infection--use lotrimin cream (Clotrimazole).
- Friction due to excess fat--you can put cream to reduce the friction but for the long-term you should work on weight loss.
- Insulin related--if you have dark patches in your armpit and back of your neck, you may have something called 'acanthosis nigrans' . it's a sign seen in polycystic ovarian syndrome and diabetes.