Did Danzo know that the Rinnegan existed? While searching for the reason why Danzo didn't get the Rinnegan, I want to ask if he actually knew the Rinnegan existed before the attack in Konoha? What are the evidences that support this fact?
He should have known before the Konoha-attack. Years before, he worked together with Hanzo, who was attacked by Nagato using his Rinnegan. Back then, Danzo most likely came to know about the reason why his men got killed.

Source: Chapter 447