2 questions really, why was he assumed dead if no body was found? Why was it assumed to be suicide?
I know one body was found but I read that dna tests showed it was a 30-35 year old woman, which was probably Eva Braun. This could be wrong though.
This answer by /u/Georgy_K_Zhukov may interest you.
Just to supplement the answer that has already been given, this was the topic of the latest AskHistorians podcast and they talked for well over an hour about this subject, without being boring! It's definitely worth listening to if you're an auditory learner.