Hello,
Pityriasis versicolor is one possibility for such dark spots on the back. It is a common skin infection caused by the yeast Malassezia furfur. This skin disease commonly affects adolescents and young adults, especially in warm and humid climates.
If the lesions turn itchy, painful, dark or starts having a burning sensation then please get an evaluation done from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Could be a number of things including tinea versicolor. Please send me some pictures if you would like me to help.
Regards
Dr.Sasi Attili
Consultant Dermatologist
www.onlineskinspecialist.com