This is one of the very common question today and it can be quite frustrating . There can be medical and non medical causes and like anything we have to find each person's cause . For each of you the cause might be different so see what relates most to you .

Under medical ;

- Hyperpigmentation ( smoking makes this worse )
- Melissa when we have dead patches
- Acanthosis Nigricans
- Bacterial infections

Not everything is medical so let's start with the easiest or Non Medical ;

- The chemical in many deodorants and antiprespirants can cause this .
- Accumulated dead skin cells - try a scrub for exfoliation but this is a sensitive area so use one for sensitive skin maybe twice a week .
- Hair roots - when you shave this is seen more and makes the area look darker .
- Irritation from shaving can cause this too . If you shave use shaving foam and shaving along the hair growth direction with a good razor .
- Wearing very tight clothes all the time .

What you can di for dark underarms ;

- Switch to a perfume
- Exfoliate
- Laser does makes it look more even toned because the black of the hair follicles can't be seen .
- Try out some home remedies .

If you feel this is medical then please see your doctor and there is no harm in doing a check up every 6 months !