Ok....let's see if I can describe the picture in my head...
How about doing a cake in something like a "fried egg" shape? A single layer, or just 2 layer, large round cake at the base, and a half-ball in the middle? That way, you can airbrush, or sculpt the spiral arms of the galaxy on the large round, and perhaps a white covering on the half-ball, with some disco dust, or something to make the galaxy's core sparkle and shine.
maybe add a few small fondant stars on wires to float around, and fill out the space above the cake?
HTH, good luck, and make sure to post pics when you're done!
Thanks for the ideas, Scott! I'm all about the sparkly stuff but I was at a loss as to how to give the cake some depth, other than an entire airbrushed mural. I'm also thinking maybe I can rig a fake center layer, using a cake circle, and insert several wires into it and bend them to make the spiral pattern. Put the half-ball on top, ice, paint and BOOM! done!