RandomCommander
...after thorough research they found out that the particles themselves seemed to be remnants of the Elder-Calamity, as the density apparently slowly rises whilst heading towards "ground zero", it appears they somehow reacted with ftl travel gaining physical matter and changing from sub- to realspace whenever something traveling faster than light passed through them, essentially ripping whatever it was apart within moments...
Though they did not seem to move by themselves, nearly as if they stopped at some point, marking the borders of the Cult-territory...
Which brought the Inquisitor-Core to the other question: How to avoid infecting space outside the borders of the Cult?
As they did not seem to move by themselves they probably wouldn´t travel outside by themselves either which, at least theoretically, left only two ways of spreading.
Any sort of ftl jump in range of the borders directly outside should, at least most likely, accelerate the particles towards the outside, even though this means the guaranteed end of the craft used for this, the other was a direct connection, via teleportation, a wormhole, spatial folding, rift travel, and similar means, though it seems the connections apparently collapse nearly immediatly, due to heavy energy fluctuation caused by the particles...
There also were a variety of other oddities, for example the particles seemed to match none of the databanks the Inquisitor-cores had at their disposal, and they seemed to react specifically to anything like ftl travel or instantaneous transport, just as if someone artificially made the particles to fulfill exactly this purpose...
Though parts of this were more of a theory...