Hi ! Thank you for your query. The darkening is due to excessive weight, hormonal issue and friction. Losing weight will help the darkening. Do not scrub very hard, it will make the skin more dark. Wear loose fitting cotton clothes. Avoid using harsh soaps, perfumes directly on skin. You can try this home remedy : Squeeze crushed cucumber to extract the juice and then add the lemon juice and turmeric. Stir well and apply on the affected area. Leave it on for 15 minutes and then rinse it thoroughly with water. Do this daily or every other day. Do not apply if there are cuts or scratches. Hope this was helpful. Dark underarms is a fairly common problem many people face. Hormonal changes and weight are the primary causes of dark underarms. Also with the changing lifestyle, deficiency and poor nutrition, more and more people are having dark body folds. Those who have poor digestion, or have thyroid problem, are overweight, use lot of deodorants and perfumes directly on skin are more prone to have dark underarms. Skin of the underarms is relatively thin and sensitive. The best treatment to treat dark underarms is to keep it clean and avoid spraying chemicals directly on the skin.The underarm hair needs to be removed frequently, albeit gently. Many topical creams that contain kojic acid helps to reduce the darkness but they should be taken after consulting a good dermatologist only. We as  Dermatologists do advanced peels and  laser treatments like, for  faster and long lasting results.