I just found a hard almost golf ball sized lump where my rib cage splits. what could it be?

Lump: I need a better description of what you mean by where my rib cage splits. If you mean in the front along the midline at the base of your breastbone this may represent the xiphoid process which is a bony prominence that can be quite palpable in some people. In any other location this could be a bone callous or something superficial to the ribs such as a cyst. I would recommend you see a physician for a full history and physical examination with possible imaging.