Obi-wan heard of Sifo-Dyas. When Lama Su mentioned Sifo-Dyas, he was taken aback by the name. He was a real Jedi. He just didn't know that he did this, since he assumed that he died before then. Lama Su acted like Sifo was still alive, yet no one had bothered to correct him. So he takes the task in hand. Dooku, a Master Jedi, erased Kamino after killing Sifo-Dyas as a test of loyality to Sidious. Once he became Darth Tyranus, Dooku returned to the Temple and erased Kamino. One of thirty some planets erased, including Dagobah. After the Battle of Geonosis, Yoda returned to Kamino to investigate things further. All documentation provided to him, was convincing enough to make Yoda believe Sifo-Dyas did it on his own. Jocasta Nu investigated and discovered that it was Dooku who erased Kamino. While Yoda went through each missing planet, he filled away the knowledge regarding Dagobah.