Do acne dark spots fade away eventually? i've had dark spots on my back and chest for 6 years.

Yes: This postinflammatory pigmentation should be gone about two years after you stop having acne. All acne can be well-controlled with today's rx's. If it bothers you, you may try a quinone bleach; most people will not think it's anything to be concerned about, but rather something distinctive to you. I hope that your primary acne problem is being managed as it should be.