What could be cause of hard lump, size of gumball, on bottom right rib felt on back/side. Had 12 years now. Not usually painful unless pressed on alot?

It sounds like. Somelike some type of cyst, maybe a sebaceous cyst but to be there so long without a secretion is difficult to say. Did u ever fracture the rib? Could it be repair of the bone from a fracture?
NotSeriousGetChecked. You have had this for 12 years and it has not changed.It is hard lump to feel which means it is likely to be in the rib itself or over it Could be a cyst or a healed scar from old fracture or a benign tumor It needs to be evaluated by your Primary Care Doctor when you next visit him/her let him check it.
Good prognosis. This sounds most likely like either a cyst, which is a pimple deep in the skin. Or it may be a lipoma, which is a fatty growth. Both lesions are benign. If they become painful, or the cyst gets infected, they may be removed with local anesthesia in the office. However, to be sure of the diagnosis, you should see your dermatologist.