Dear lybrateuser,
-Dark chocolate is good as it contains cocoa extract which has antioxidents with beneficial affects on the body
-do have in limited quantity about 2-3 small pieces per day, excess is harmful as it will raise the blood sugar.
Don't eat it more than once a day. Clean your teeth properly after you eat or drink anything. Get you teeth x-rayed to determine the extent of decay. Get your cavities filled accordingly with or without root canal treatment.