What Causes Dark Spots On Back And Shoulders?

hello i am 26 and i am a black male. i am dark skinned and now i have dark spots appearing on my back and shoulders. ive seen light skinned black people would have light spots appear on there skin. what is this? i was thinking of maybe because of drinking to many drinks thay contain acidic in it. could this be and what is a cure for it? i was told to try selsun blue