The Grand Finale Galaxy is unlocked when beating the game with Luigi with 120 stars. Talk to the Luma and say that you want to go to the Grand Finale Galaxy.
This time Peach's Castle is back to the Kingdom because you defeated Bowser three times. The Grand Finale Galaxy gives you a chance to get the 121st and final star by collecting 100 Purple Coins. Once you get 99 out of 100 Purple Coins, the one Purple Coin remaining is on top of the Starshroom ship. Backflip up there to collect the last Coin. Get the Star at Peach's Castle to finish the game. Talk to Mailtoad to send a picture to the Wii Message Board congratulating you by how much times you lost a life during the game. Now you can play any galaxy you want in any dome you want by just getting all silver stars.