What Is A Dark Patch Of Skin Between The Breasts Suggestive Of?

Hello, For as long as I can remember I have had a dark patch of skin I between my breast. Eczema is rampant in my family and I thought is was that for years. However otc eczema creams have never really had an effect on it. even got prescription eczema creams. Nothing. During a breast exam by a new doctor a few months back, the doc showed some conceren. She thought it was yeast. Or fungal in nature. But I told her it neither itch, burned or irritated me other than aesthetically. She was unsure and noted that the skin had thickened over the years and that s why it is so discolored. She prescribed clobetizol prionponate. It did have a bit of a positive effect but caused the skin to still look a little rough but now bleached. I am black, and the bleached patch was worse than leaving it be. I ve sense stopped using the cream and the dark patch has returned to its originall state. Noting that am predeiabetic. I ve seen some research linking the two, but nothing concrete. What else could this be?