It's not the edge of the galaxy - if you're enroute to Colonia, you're nowhere near the edge.
Some of these "cubes" that you see are actually there; they represent boxels of large bright stars, surrounded by boxels of fainter stars. Obviously the real-universe galaxy isn't made up of cubes of stars like that, but the ED galaxy is - it's just a quirk of the way the galaxy was procedurally-generated, and there's not much that can be done about those, short of deleting the entire galaxy and starting again.
But since you're in the vicinty of the Galactic Core, I suspect it's a rendering issue. There are far more stars visible in the Core, than out near Sol where most of the playtesting was done. Every star you see inthe sky is a star you can travel to in-game; this means that for every single star in the skybox, the game has to calculate where it is, and how bright it should look. Sometimes, in star-dense regions like the Core, the graphics engine simply says "Nope, this is taking too long, I've drawn enough stars today, no more stars for you".
Try rebooting the game. If the "boxes" disappear on reloading, it's the second issue. If they're still there on reloading, it might be the first issue. I know many times, these "boxes" ruined a nice screenshot; on reloading, they went away.