You need a very stable substrate for this technique, or the large expanse of grout will tend to crack. I have used the "Marmox" board for this piece, as it has less tendency to warp than anything else that I'm aware of.
This detail illustrates the space between galaxies; you feel you want to stare into the void and lose yourself!
Of course I've used my trusty Ring Saw to cut the "galaxies".