After swelling goes away dark spots take place of them. my complete back is covered with dark spots . Urgent help required.
If I understand your question right, the problem is not acne by itself but dark spots or postinflammatory hyperpigmentation after acne has gone.
Here is one medical article (Emedicine) that describes the treatment. The treating substances include hydroquinone, tretinoin cream, corticosteroids, glycolic acid (GA), and azelaic acid. I don't think you should try these treatments on your own; ask a dermatologist about it.
I personally do not believe that home remedies would help, because dermatologists and studies do not really mention them.