A while back, I decided I wanted to make an edible galaxy. Like you do.
I wanted to give the illusion of a galaxy suspended in space. The idea was to create the top half of the galaxy, and place it on a mirror. Here’s one of the early failed attempts.
After a few tries involving various types of bakeware and reflective surfaces, I hit upon this.
The galaxy core is a dome-shaped cake, baked in a shallow bowl, and frosted with white icing. The spiral arms are a combination of white M&Ms and an sugar. I actually found a place that sold white M&Ms in bulk, which saved a lot of time picking though mixes.
To give the illusion of it being suspended in space, I arranged the whole thing on a sheet of reflective Mylar, and created a black backdrop out of presentation board.
If you dim the lights a bit, and view it from the right angle, it looks pretty good.
One refinement might be to replace the backdrop with a large-format printout of a starscape, giving you a result something like this.
If you’re interested in making one yourself, there’s a tutorial here.