Thanks for sending a photo. A bit about my background… I have been an Esthetician for almost 9 years and in school currently to become an electrologist.
Pore (hair follicle opening) sizes vary from person to person whether it be on your face or underarms etc.
Because your photo is rather up close it is hard to get a sense of what the skin looks like overall to see how many look like this in that area. No concrete way of knowing just what the exact cause is unless you had before photos before you started using the epilator and this occurred as a result shortly thereafter.
Pore sizes in the area are known for being larger and I quite often see clogged pores that result from the accumulation of deodorant, sebum etc. Sometimes they can get quite large and even need to be expressed. But that’s not what I am seeing here though.
I agree with Michael that it’s likely scarring in the follicle itself.
Epilating by tweezing or waxing that is not quick enough puts more traction time on the hair.
The best way to describe it is when you see a magic show and they have a table set up with beautiful dishes on it and they pull the tablecloth very quickly and the dishes remain. Pull the tablecloth slowly and the dishes would all fall off.
In simple terms is when applying traction on a hair you are pulling the hair and the cells attached in the follicle with it. The faster the epilation the less internal traction time. Which is why in waxing you are taught to pull the skin taut and pull the wax quickly and I mean really fast… do it slow and it is 3 x more painful and is more likely to result in broken hairs, skin lift or suction bruising.
Without getting my eyes on your skin in person my opinion is that there is some hyperpigmentation and damage within the follicle itself that is causing an unwanted external appearance. Part of the darkening could just be the shadow of the pore itself. If you are epilating very regularly it may stay in this condition perpetually because it can take a while for hyperpigmentation to resolve so when you are doing it every month it just doesn’t have a chance to heal itself fully. But it could also be permanent.
You could alway stop and see how your skin looks in 6 months but then you’d likely have a bunch of hair and not be able to even tell.
Electrolysis would likely be a good option for you if available in your area to destroy the hair follicle for good so you can "be done with it " so your skin can fully heal. As to whether the appearance would resolve no one can fully predict that for you.
I have clients who have been waxing for decades and their skin and pores are flawless, not enlarged at all and then there are the few that struggle no matter what form of hair removal they use. What our cells do has a lot to do with age, genetic makeup, hydration, exposure to the sun, overall health, condition of the skin etc.
The great thing about underarms is we don’t walk around with our arms overhead so it’s not an area a lot of people would see or even take notice.
If you wanted to, you could upload another photo that is not quite so close up to show the whole area but I do think the answer would likely be the same.
I hope this helps you!