I have a dark rash that appears in almost perfect circles. what could this be?

"ringworm" is fungal: Infection of skin (tinea corporis) [no worms involved]. Starts as point & grows outward, giving ring appearance. Edge slightly raised, may be slightly scaly, center looks normal. Treat with otc antifungal cream like Lamisil (terbinafine) until all lesions gone at least a week. Check pets as source of fungus. Athletes can get it from gym mats, which should be sanitized regularly.