A few causes of dark underarms include shaving which leaves a dark stuble, an allergic reaction to deodrants, or wearing tight clothes which rub and irritate the underarm.
Dark underarms may be due to Friction of to tight blouses and tops that brush against the skin, Excessive sweating, Infections due to improper washing of underarms, and shaving.
If you have dark hair that may be the cause, stubbly hair tends to make skin look darker as it grows or maybe you just need to ex foliate, there are a lot of ex foliating scrubs out there you could try or look for a simple home made sugar scrub
