My dentist cleaned my teeth 6 days ago.since then, there are some grey/dark lines where the sides/bottom of two teeth meet the gum line.what are they?

Grey lines?: It's hard to know for sure what's causing the grey lines sight unseen or without at least a photo . Speak to or go to your own dentist for a definitive answer and if there is anything that can be done to remove them.

Crown?: This could be the margins from a crown, dark roots or a cavity that is now visible after cleaning your teeth. This is something you can address easily with cosmetic dentistry. Without a picture and /or an exam, it's difficult to say; but your provider would probably be able to give you a quick answer--maybe even over the phone. Call and ask or set up a follow-up visit.

Grey lines: Sometimes when teeth are cleaned inflamed gums return to normal size, exposing tarter that was trapped under gums. Sometimes fractures in enamel are revealed. There may be other causes. Please return to your Dentist for reevaluation.