The Scattering was a major historical event that occurred in the chaotic period following the death of Leto Atreides II, the God-Emperor. Leto's death saw the eventual breakdown of his empire, severe famine on many worlds, and the introduction of Ixian no-ships. It was all part of The Golden Path, as put forward by Leto, which was triggered by a perceived need to diversify the human race and allow it to evolve.
Leto II's prescience allowed him to see into both the past and the future, and he believed that ten thousand years of rule by the Faufreluches had allowed the human race to become stagnant, and that drastic action on Leto's part was needed in order for the human race to survive.
The entire purpose of "Leto's Golden path" was to force the human race to abandon certainty, security, and safety as individual goals and to "scatter" not only geographically, but also to do so technologically, ideologically, and socio-politically.
As a result of this philosophy, during the reign of Leto II humanity was predominantly planet bound and ruled by an iron fist for more than 3500 years. This was in contrast to the relative freedom of expression and movement people enjoyed to a limited degree under the Faufreluches, the limits being predominantly the inherent control of feudal rule and the Spacing Guild monopoly on space travel. Indeed, Leto II exerted a level of control that was unprecedented in the Imperium, including power over the Guild and space travel, a feat that previous Emperors had been unable to accomplish. Upon Leto's death, the stalwart and long-established order of the Imperium disintegrated, as did control over freedoms and movement, and as a result, humanity rediscovered independence and nobilities on an unprecedented scale. This paved the way for humanity to "scatter" across the universe into the unknown and further evolution of the human race.
According to Leto II's plan, his death was followed by a collapse of political control and Spice supplies - as it would take generations for the Spice cycle to reassert itself on Rakis. Political order collapsed and entire worlds starved to death, in a chaotic period known as the Famine Times. The Scattering was an immediate response to the Famine Times, as refugees fled the anarchy consuming the Old Imperium.
During The Scattering, many people remained within the boundaries of the Old Imperium. Many, however, did not. Countless humans departed the Old Imperium for uncharted space, never to return. For 1,500 years, humanity explored the uncharted realms of the universe beyond the boundaries of the Old Imperium. New groups and cultures evolved.
During the Scattering, vast waves of refugees spread outwards into uncharted space, so that the number of human-colonized worlds exponentially ballooned both in number and spread. The Bene Gesserit who remained in the territories of the Old Imperium could only make vague guesses at the size of "Scattering space", as it was theoretically infinite and even the Honored Matres might not know its full extent. One clue is that the Lost Ones returning from the Scattering refer to the Old Imperium as "the million worlds" or "the million planets" - implying that Scattering space is at least several multiples larger than that.
Little is also known of the exact socio-political history of what happened in Scattering space, other than that its development across 1,500 years was fairly rapid - a direct reaction to the enforced sedentary nature of Leto's Peace for 3,500 years, or the 10,000 year static structure of the Faufreluches under the Corrino empire.
What is known is that eventually, the Honored Matres developed as a fusion of Bene Gesserit and Fish Speakers (and possibly other groups) who had fled into the Scattering. An aggressive group focused on conquest and domination, the Honored Matres carved out vast swaths of territory among the worlds settled in the Scattering. One of these worlds, or perhaps a collection of worlds, was referred to as "Dur".
Another group that rose to prominence in Scattering space were the Lost Tleilaxu - no longer bound by even the pretense of following the restrictions of the Great Convention, they pushed genetic manipulation drastically farther than their Bene Tleilax forbearers in the Old Imperium ever did. Among other creations, the Lost Tleilaxu brazenly spliced together human and animal stock, creating the feline-based Futars and fish-based Phibians. The Honored Matres came into conflict with the Lost Tleilaxu, becoming their bitter enemies, in wars that spread across much of Scattering space. During this time the Tleilaxu refined the Futars, creating them for the express purpose of hunting Honored Matres. Nonetheless, the Matres managed to capture and enslave many Futars and Phibians.
Return from the Scattering
About 1,500 years after the death of God-Emperor Leto II, many of the Lost Ones began to return from the Scattering to the space of the Old Imperium. Chief among these were the Honored Matres, who carved a path of conquest wherever they went, and exterminated the Bene Tleilax and Bene Gesserit wherever they found them.
The Bene Gesserit eventually learned that the cause of this return was that the Honored Matres were fleeing an even more powerful adversary they encountered out in the Scattering, which the Honored Matres only briefly alluded to as "The Enemy" or "the Enemies of Many Faces". The Matres had managed to steal some of the powerful Obliterator weapons from this Enemy before being easily overwhelmed. While the Honored Matres were fixated on destroying the Bene Gesserit, they were also determined to learn their methods of cellular control over their own metabolism which granted them immunity to disease and poisons - a skill that had atrophied among the Matres themselves when they split off from the Bene Gesserit. From this the Bene Gesserit also deduced that the Enemy the Matres were fleeing had made extensive use of biological warfare, against which the Matres no longer had adequate defenses.