What is this black or grey stuff at on the top of my back teeth in the grooves?

Sounds like you are: Using a mirror to look at your lower back teeth. Your description suggests you may be seeing: dental cavities, failing fillings, leaking sealants, amalgam fillings that look black/gray, black/gray food stuck in the grooves, or a combination of some the above. Get to your dentist and ask this question. None of us like to see black/gray on our teeth! glad you are paying attention to your teeth!