Bowser Jr's Robot Reactor is a galaxy featured in Super Mario Galaxyas the final galaxy in the Terrace. It is a pretty short galaxy, due to it only having one level which is the first major required boss.
In the level, you will arrive at the galaxy's hub, and after using a Bullet Bill to break open a cage containing a Launch Star, you will be able to launch yourself to a moon where you must fight Megaleg. After defeating Megaleg, a Grand Star is collected and not only is the galaxy complete, but it will unlock the Fountain.
A glitch was discovered in this galaxy which is if the player triple jumps on the glass cage containing the Launch Star the gravity will pull the player to Megaleg's planet, expect that Megaleg is unactivated. If the player walks on the brown square on Megaleg's planet, the boss battle will start.
If the player looks at the back of one of Megaleg's legs, they'll find three cages holding various prizes.