I am afraid but wanna tell you that please be proactive about finding it's cause because It appears you are having Acanthosis
nigricans- a telltale epiphenomenon of Visceral Cancers like that of Pancreas
and Gut, Liver
,breast, brain etc.
Rarely they may be caused by rapid gain of weight or highly stressed life style.
Also consider to stop some recently started medicines as a possible cause.