Frank Herbert's son later teamed up with a sci-fi author and published some books which wrap up the story and also explain some of the events that happened before the Dune books take place. Supposedly from his father's notes. Not everyone considers these books canon. The catastrophe, however, is revealed but at this point it mostly seemed the first book had some parallells with the Middle East.
The reason that many don't consider it canon is that it directly contradicts not just small events in the originals but the entire premise. In Dune, the Butlerian Jihad was an ideological struggle against people who were willing to delegate their thinking to machines without considering the long-term social consequences (hmm, still seems pretty relevant) and ended up being controlled by oligarchs who controlled the machines. The outcome was an overreaction against machines, banning even simple calculating engines. In the cash-in novels, it was recharacterised as a war against a two-dimensional and completely unbelievable machine intelligence.
The final revelation in the sequels was then that this machine intelligence had survived and had been building an empire in secret all of the time that humanity had been building their own and eventually decided that it wanted to destroy all of the humans (why? Because that's what evil robot overlords do! Obviously). These books could have been written by the Bene Gesserit sister that Leto just managed to restrain himself from killing in God Emperor, for her stupidity. He explained that humanity had moved past the point where machines could be a threat (remember: they were never a physical threat, the threat was always stagnation and decay as humans delegated more and more to machines until there was no point in continuing to live).
The point of the scattering in Leto's Golden Path was that humanity would spread out so that nothing could be an existential threat (the old Empire had more or less stopped expanding and didn't have exponential growth to protect it). Part of the point of Chapterhouse was that the conflict that was going on, in spite of engulfing more worlds than the Empire in the time of Dune, was a tiny sideshow - nothing that happened would affect humanity and the descendants of humanity as a whole. The big hint about the changes that were happening out of the empire was the extent to which the Honoured Matres and Futars had diverged from what was considered human. The implication was that they were the ones that had diverged the least and were no longer able to compete with far more predatory creatures that had evolved from humans.
