MarioWiki:Sandbox

Cosmic Mario is a character that appears in Super Mario Galaxy. His physical shape is identical to Mario, while his outer appearance is blue with stars all over, as if he were made of cosmic matter. He also has an unusually high-pitched voice. If Mario enters a galaxy when a Cosmic Comet is orbiting a galaxy, Cosmic Mario appears at the beginning at the level and challenges the player to a race. The player must then race through numerous obstacles and reach the Power Star before Cosmic Mario does. If Cosmic Mario manages to reach the Power Star first, Mario will lose a life and must restart the race. Colliding with him during the race causes him to release Star Bits.

A similar entity based on Cosmic Mario appear in Super Mario Galaxy 2 and Super Mario 3D Land in the form of the Cosmic Clones. Other entities are the Cosmic Spirit from Super Mario Galaxy 2 and Mummy-Me from Captain Toad: Treasure Tracker.

Cosmic Luigi
Cosmic Luigi also appears, replacing Cosmic Mario when playing as Luigi. He is very similar to him, with the only major difference being that he takes advantage of shortcuts more often and uses a lot of Long Jumps, making him faster and much harder to beat than his counterpart.

In Super Mario Galaxy 2, despite not being in the game while playing as Luigi, Cosmic Clones behave very similarly, being slightly faster.

Trivia

 * At the start of a race, if the player holds the up, then  when the timer hits 2, and then hits  just as the race starts, Mario or Luigi will get an extra boost at the beginning of the race, shown by a yellow color appearing on Mario/Luigi, shrinking gradually as a marker of the end of this effect. This is similar to the Rocket Start from the Mario Kart games.
 * Cosmic Mario shares its voice clips with the Cosmic Clones.
 * An unused variant of the Cosmic Mario Race in Space Junk Galaxy is found in the game files. When players start a race, they start at the coordinate 0,0,0, which is the air that they land on the starting planet. Cosmic Mario cannot win on this race, and players still have to use Star Chips to unlock a Launch Star, but Cosmic Mario does not need to use the Star Chips to use a Launch Star. Cosmic Mario does not use voice clips in this variant.
 * Cosmic Mario/Cosmic Luigi's theme uses a similar arrangement to Shadow Mario's theme of the same music, the underground theme from Super Mario Bros..