Black hole

A Black Hole is a very dangerous object found in Super Mario Galaxy that acts as an obstacle which can suck Mario (or Luigi) into it if they are unlucky enough to fall off of some ledges or miss the target when using a Sling Pod, etc. It's slightly similar to an Abyss in Mushroom World. If either one of the plumbers is sucked into the black hole, they will lose a life. Black Holes are usually impossible to avoid once the player has fallen off a platform and into outer space, though this obviously does not apply if Mario is in possession of a powerup that grants him the power of flight, or something along those lines, such as the Bee Mushroom, the Boo Mushroom, the Red Star, or the Cloud Flower. Black Holes will suck the player up extremely quickly once they have entered the immediate vicinity of the Black Hole, of course depending on the size of the Black Hole as well as how close the player is to it. For this reason, these objects are arguably the most deadly and/or dangerous objects in the entire game.

However, Mario or Luigi can also use Black Holes to their advantage. For example, any enemies or objects that fall into a Black Hole will be destroyed after falling a certain distance toward it, so Black Holes can sometimes be used to destroy certain enemies in various areas. Note that any enemies destroyed by falling into Black Holes will not yield any items whatsoever.

At the end of the game, after Mario has defeated Bowser for the final time, his gigantic sun will collapse into a massive Black Hole, which will quickly begin to engulf Mario, Princess Peach, Bowser's entire fleet of Airships, Princess Peach's Castle and the Comet Observatory. Just as everything is about to be destroyed, Luma suddenly appears in front of Mario, waves at him and then sacrifices himself by throwing himself into the Black Hole. Thousands of other Lumas soon join him in flinging themselves into the massive Black Hole in hopes of stabilizing it and ultimately saving the universe from destruction. Eventually, the Black Hole twists wildly around in space before finally exploding in a new "Big Bang", thereby reforming the entire cosmos and creating an entirely new galaxy, which Mario welcomes in the final cutscene of the game.

Trivia

 * A Black Hole also appears in the Mario Party 6 mini-game Black Hole Boogie.
 * In Super Mario Galaxy 2, the background of World 6 appears to be a giant black hole. As such, this is where some of the toughest galaxies in the game are located, along with Bowser's Galaxy Generator.