Dark Matter

Dark Matter is a dangerous hazard that mainly appears in the Super Mario Galaxy games. If Mario or Luigi touches Dark Matter, then he instantly dies even at full health. Dark Matter tends to eat through platforms causing Mario to jump over them to avoid damage. Dark Matter is also used in some of Bowser's attacks such as the Dark Matter Spin.

Super Mario Galaxy
Dark Matter mainly appears on Bowser's Dark Matter Plant. If Mario or Luigi touches the Dark Matter, then they instantly dies. Dark Matter could also be found during the Purple Coin mission on the Toy Time Galaxy where it replaces lava on the Luigi side.

Super Mario Galaxy 2
Some Dark Matter could be found on Bowser Jr.'s Boom Bunker. The same rule applies since the last game as being a hazard that instantly defeats Mario or Luigi.