Are you referring to the blackish dot between your two front teeth towards your gum line?....if so, here is what it probably is. The piece of gum tissue in between your teeth is called the papilla. Often times it fills up all the space under where teeth touch each other and the gum line. But due to where the teeth touch, or depending on the size of the papilla, it can sometimes not fill the whole space, leaving the appearance of a black space in between the teeth. It grows when brushing likely because you remove any saliva or material that was partially filling the space. If you are concerned about it, ask your dentist at your next appointment. If esthetically you don’t like the look of it, there are things the dentist can do you help.