OK, my muse just whispered this to me so it's totally unrefined...
...though refining further as I type, and the more I type the more I like so my sig below definitely applies, feel free to use or borrow the idea for you own use with credit but you may not claim it as your own, either as is, expanded upon, or altered...
Scene: A top secret research station somewhere in the Imperium...
The experimental Reflex Jump Drive had shown great promise in hundreds of simulations and the first prototype was ready for an actual field test. If successful it would revolutionize jump space travel by opening up the higher realms only accessed via misjumps before now. With a huge savings in fuel requirements and the ability to predictably travel up to 36 parsecs in a single jump this could be the biggest breakthrough ever in the field of jump space travel. The clock counts down...
...a blinding flash and the test ship 100d from the planet disappears. Cheers go up for the apparently successful jump. Instruments at the research station record an anomalous energy spike seconds later, just before the planet disappears like the ship. Remote observation ships in far orbit record a similar spike, scramble to jump clear but also wink out before they can power up their jump drives...
About 7 years later the primitive natives of a nearby world wake up to find that one of their constellations is missing a prominent star. By then of course the Imperium, such as it is now, is already aware of the missing star system, though it is deeply covered up, and is still experiencing the effects of that day. It, whatever it was, spread swiftly at about 1 parsec per week and is continuing to expand. The effect, dubbed thick jump space has resulted in a drastic reduction in the ability of ships to jump. Range of jumps has been reduced 6 fold such that only a J6 drive can travel a full parsec, and the time spent in jumpspace was also affected by a like 6 fold increase, so all jumps now last 6 weeks.
The immediate effects were devastating in the short term. Initially uncounted ships disappeared, presumably jumping short of their destinations by the 6 fold range or being lost entirely when they ran out of power to maintain jump because of the 6 fold extended time in jump space.
The long term effects were catastrophic. Warnings were impossible to send as the effect spread as fast as ships not already caught up in it. Word only came months or years later, in most cases to worlds already dead for lack of supplies from trade ships.
With only a few J6 ships able to cross interstellar space in reasonable time, and that without much space for cargo, interstellar trade ground to a halt. Without trade many worlds died, many many more became independent. The dream of empires of stars died. A few ships still cross the new gulfs between even close systems but they are strictly military and vital supply ships linking small clusters of no more than a few systems. Even intersystem travel is now faster in real space than jump space.
After decades the effect is wearing off slowly, beginning where it started and expanding. In time it is believed jump space will return to normal and dreams of empire are already growing in the minds of some...
...the future beckons.
Anyway, there it is in a nutshell. And all that from two words (more the concept than literal) whispered by my muse, "thick space" and me typing as fast as I could to get the whole gestalt out before it faded

Now it looks kind of familiar and I'm not sure if it's because the idea blossomed full into my mind or because I've seen it or similar somewhere before

So if anyone recognizes it do let me know the source. If I copied it in any part or whole it is entirely innocently and I honestly don't recall ever seeing it before.
Sooooo, comments?