How did obito not die?

well, i dont think so... but all i have is a theory.oh and if you dont know who obito is... he was in kakashi's squad andwas an exceptional uchiha. yes i said he was an uchiha. he supposedly died on a mission. this is where my theory comes in. so my theory is that zetsu ( leafy akatsuki thing) found the dead-ish body of obito and had kakuzu ( immortal-ish akatsuki) sew him back together. then he put madara uchiha's soul in the body. and that explains his double personality. currently he is in the akasuki and his name is tobi. obito+madara=tobi