So, while contemplating the fluff of my WAAAGH!!! and the sub-sector it affects, I came to the realization that Gork's Grin, or the Great Rift/Cicatrix Maledictum to you non-greenskins, would not be visible to most of the galaxy. After all, our galaxy is like 105,000 light years wide, so it would stand to reason that after 200-400 years (or whatever it's been), only a small fraction of the galaxy on either side of the rift would have light reaching it for it to be visible.
Or would it? Afterall, it's literally the realm of nightmares, chaos, and unreality spilling out across realspace. Could it be visible by something other than light?
Assuming it isn't visible by weird, 40k-not-magic-but-come-on-it's-magic logic, does most of the galaxy even grasp what has happened? Anything that's touched on this? I just started Spears of the Emperor, which I know is on the Nihilus side of the Imperium, so please avoid major spoilers from that if it covers it.
A little more of my thought process: My fluff takes place in a sub-sector on the far edge of the galaxy, out near the Ghoul Stars. While thinking about how the chapters involved in my story would take Primaris, I realized that they probably would have no idea that the galaxy is torn by a rift. They'd know from Navigators and Psykers that the astronomicon was gone and communication was difficult, but if they had a moment with Custodes like the Blood Ravens do in the story in July's White Dwarf, would they even know about the rift? Would it just be a rumored thing, passed on by the Primarch's representatives? Would they believe it, or grasp what it is?