There's this black stuff at the bottom of my lower front teeth... there's no pain at all . what is it?

Could be...: Many things can stain black in the mouth, namely coffee, tea, leafy greens, and most likely calculus. No not the high school class but rather a hardened colony of bacteria. It is best to stride into your dentist's office and have them diagnose it for sure. Diseases in the mouth are often so low on the pain scale that we tend to ignore them until it's too late. Please don't ignore it.

Decay, stain, calcul: This could be stain, decay or calculus(tartar). Stain or calculus would be removed by a dental cleaning, but decay would require that a restoration be placed. Have your dentist check this out to guide you to the right solution. Good luck.

Few possibilities: Black stuff can be a few different things, all of which would need professional attention to be addressed. You may have calculus which commonly builds up on lower anterior teeth. It could also be tooth decay (which may or may not be associated with pain), it could be a stained margin from an existing restoration, or finally the metal margin from a crown if that tooth happens to have one. Good luck!