Ah, yes! If I travel a galaxy's length from the end of a galaxy (having enough fuel after beating the last boss in a galaxy), I can trigger another boss battle!
Too bad no ship is actually at the end of the galaxy I was on when I tried this, though :)
That would have won you an 11/5 stars.
Overall, I thought it was a great game. Not very "deep", but addicting. Not in a bad way. It's relaxing. It's pretty clear that the player's expectations shouldn't be too high, which means that the player can choose to relax rather than stress over trying every possibility the game has to offer, hoping for some kind of intended result.
Nope, pretty straight-forward.