Five millennia ago The Enemy maneuvered access through several Gates, spewing forth the Dissolution plague. Trillions died in the Chaos which ensued.
The Dissolution was, or rather is, a plague of hydrocarbon phages artificially developed by The Enemy (an unholy alliance of several unnatural AI (Artificial Intelligence) powers). The phages consume and dissolve plastics and their derivatives.
The phages dissolved the materials of civilization. Even artifacts primarily of other materials were affected; building of seemingly impervious stone had their plastisteel pins and bolts, plasticrete mortar and plastering dissolve. Appliances from watches to space ships dissolved, leaving only a mess of wires and gears, glass and crystals. Even natural fiber cloths fell apart as stitching of polyester thread disintegrated.
The Dissolution washed over vast regions of The Harmony. In the infected regions Cosmic Civilization collapsed as its materials melted away. Thousands, possibly millions, of worlds were thrown from civilization to barbarism, a fall known as The Chaos.
The Harmony had no choice but to abandon the infected areas until decontamination procedure could be developed. Gates and Links were purposefully broken, cutting contact between infected regions and those yet healthy. A much-reduced Harmony sheltered behind sealed Gates, Cosmic Civilization safeguarding Organic Life as it continued its valiant efforts in The Great War.
Beyond the Sealed Gates is Tsepitslen Melukhes, the "Shattered Realms" of the Onvern Velt (Lost Worlds.)
Some once-rich worlds went lifeless. On others, sapiens clung to the shredded tatters of civilization and rebuilt. On yet others, the fall was complete; civilization may or may not have ever revived.
Tragically, many of the worlds beyond the Sealed Gates were never actually contaminated by the Dissolution, but, in those desperate days, even suspicion of contamination meant severing of ties.
As the Quarentine was never perfect, sucssesive wave of Dissolution plague washed more and more worlds from The Harmony into the Tsepitslen Melukhes.
As a result, some of the Onvern Velt were lost a full five millenia ago, but others were lost as recently as five centuries ago.
As The Harmony finds ways to deal with the Dissolution phages, engineers attempt to revive those old Gates and Links that were not utterly destroyed. There are also projects to build new Gates and Links.
Clean Worlds
On most worlds, once the Dissolution phages ran out of fuel they died off. Such worlds are open for normal evaluation and colonization and / or fostering of any Indigenous culture(s.)
Quarantine Worlds
On many worlds Dissolution phages remain active. Where they are in their original form, they can be neutralized and the world re-classified as Clean.
Where there have been mutations, required treatment varies. In some cases, decontamination procedures can be developed, and contact resumes, abet with some hindrance and expense. In other cases, more dangerous mutations of the phages requires a Crimson Zone status and complete enforced quarantine.
For an example of a region where The Harmony is reclaiming a portion of Tspitslen Melukhes, SEE Tsvayg Tsurik Oyfshteln, "The Branch Restored."