Molar tooth looks black

Hello community. I was horrified to find that my molar tooth looks very dark, almost black from the inside. So if you look at it from above you see it, not from the side. I do not know what that means now, because I have no pain at all .. Of course I will go to the dentist .. But still I want to ask what can come from. I actually brush my teeth often .. I can only imagine that it has to do with the plastic tooth splint .. It would be helpful if some of you are familiar with it and can calm down a bit .. At 19 I still don’t want to lose a tooth: ((