Tooth probs. I do brush my teeth 2 times a day

at my last cleaning my dentist simply said this was plaque on the back tooth due to my molar not being up at the same height as some of the other teeth. Therefore making it harder to clean. I have no sensitivity and it doesnt give me any problems. The pic makes it look blacker than it actually is. Could it be a cavity or is it just plaque like my dentist thought. I go back in a few months for a cleaning as well.