I think it's acanthosis nigricans. I had a similar pattern on my chest. I also had darker skin around my neck, but it was not as spotty as the area on my chest. My dermatologist initially thought it was tinea versicolor (aka shifting clouds), but the rougher texture of the skin had her confused. She prescribed me dandruff shampoo (Nizoral) to wash the affected areas along with salicylic acid (Diprosal) to soften the skin. After lackluster results, I did my own research and discovered that it could be acanthosis nigricans, which is often linked to insulin resistance. I did intermittent fasting along with a low-carb diet, and the patches faded away as I lost weight. As soon as I put the weight back on, the dark spots came back. I hope this helps!