This is the final level in the game. To get it, you'll have to find all 120 stars with both Mario and Luigi. Once you do, you'll be able to travel back the Mushroom Kingdom for one final challenge. It's a purple coin challenge, though thankfully there's not time limit and all of the coins are extremely easy to find. There is no time limit, but there is one offcentered coin. You can do this level with both characters, so get to work!----
After beating the Grand Finale Galaxy with both characters!
Once you beat this galaxy with both characters, one new feature will be added, and while it's not much, just be glad that it's something. Once you've done so, you'll be able to see how many times you've died with each specific character on the main screen.