It would be impossible for anyone to guess what it may be. If it doesn’t disappear in a few weeks or it becomes irritated, inflamed, or itchy, you should probably see your doctor for evaluation/follow up.
Hi, does it look like this photo, slide #3? (though that one, acanthosis nigricans, is darkened a little and yours is not)
Maybe yours has a similar cause. The main suspect in that darkened one is diabetes. After that, it might be that something else is growing inside and the skin effect is a sign of what's on the inside.