Okay, I have looked this up everywhere and I simply cannot figure out what it is so I figured this would be the best place to ask.

Every since I could remember I've had this weird brown stuff in the crevices of my molars... only on the last molar (on both sides of my mouth). I have been to the dentist a million times in my life and had my teeth cleaned, etc, but none of that gets rid of this stuff. Brushing obviously doesn't do anything either.
Anyone know what it is and is there a way to get rid of it? It just looks gross and I would love to figure out a way to get rid of it.