This galaxy is available once you have access to the Planet of Trials, which is unlocked by collecting the three Green Stars in levels Luigi under the Saucer, The Secret of Buoy Base and Treasure of the Pyramid. This galaxy appears to be a trickier version of Loopdeeloop Galaxy.
The Galaxy’s Greatest Wave
The aim of this level is to complete the long and winding race course. You have three minutes to complete the course, and the default best time is one minute and a half. There are no enemies or anything hostile that will damage you, but falling off the course will cause you to begin the race again.
To ride the waves on the back of the Ray, you’ll need to use motion controls to tilt in whichever direction you want to go. You’ll also need to press A to accelerate, though you don’t always need to be pressing the A button. In fact, you’ll want to be careful when to hold down A for a big acceleration and when to only tap A to slow down.