Had a cavity the entire tooth on outside is almost gone n is black gums arent swollen red or swore so will it just basicalt dissolve or fall out?

Unlikely: The tooth with deep cavity is full of harmful bacteria. Unlike a baby tooth, a permanent adult tooth won't fall out so easily, as the roots are much deeper. For now, your body is constantly fighting against ever-increasing bacteria in the tooth. Left untreated, you will likely have an abscess--a dangerous infection that can invade other parts of face and body. Please see a dentist asap.

Not advisable: As the tooth decays, the nerve will be affected and probably die well before the tooth decays to nothing. Chances are you will get an abscess and need to be treated as an emergency... An a dentist's office. Best bet is to call now for an appointment.

Probably not: The root of the tooth, 2-3 times the length of the crown that has decayed, is still present. Decayed tooth is a focus of infection, and infection can be quite serious, affecting other organ systems. Please see Dentist ASAP before this becomes life threatening.