There are several causes of dark underarms, many of which can easily be resolved. The first cause of dark underarms is shaving. Because shaving only cuts the hairs at surface level, if you have hair that is slighlty darker than your skin color, it can give your armpits a dark color. A simple solution to this issue is to wax or pluck your armpits instead of shaving them.
Another cause of dark underarms can be a build up of dead skin cells. A way to solve this problem is to exfoliate with a sponge, exfoliating soap or body scrubs.
Dark underarms can also be caused by an allergic reaction to your antiperspirant or deodorant. Sometimes if you have an allergy to an ingredient in your deodorant/antiperspirant, your skin my react with a discoloration. Use deodorants that contain only natural ingredients instead.
Dark underarms can also be caused by a constant rubbing caused by tight clothing on the underarm. Try wearing looser fitting, cotton clothing to reduce your dark underarms.
Type II diabetes can cause discoloration and darkening of the underarm and neck area. If you think this may be the cause of your dark underarms consult your physician.