Whitening products have been used to have fairer skin. But although some people have naturally fair skin, the underarm remain to be darker than the rest of the body. The darkness may be caused by any of these factors:

Under arm hair growth – shaving once a day is not enough, since you will notice that hair has grown a few millimeters at the end of the day.

Residue from products like soaps, deodorants and creams accumulate when the underarm is not totally cleansed. This may darken the skin, and form dark lines along the folds.

Plucking can leave some scars and marks that look dark from a distance.