Hello. As ur query is regarding dark underarms, u should note down few things. Is it jus the color that has become dark?? if so it might be familial or infection or nutritional deficiencies etc. If the skin has also thickened along with the darkness, then u should see over nape of ur neck and knuckles.. If that too is thickened and dark then it might be "ACANTHOSIS NIGRANS" . Its a condition that's commonly run in families or with family history of diabetes or because of overweight .. If so, weight reduction and regular sugar check up needed.
And for treatment , there are creams and oral medications according to the underlying condition . For better and faster results, laser treatments too help.