Dark Matter is a dangerous hazard that mainly appears in the Super Mario Galaxy games. If Mario or Luigi touches Dark Matter, then they instantly disintegrate, effectively killing them, 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, however, it only does one wedge of damage.
Dark Matter mainly appears in Bowser's Dark Matter Plant. If Mario or Luigi touch the Dark Matter, then they instantly die. Dark Matter could also be found during the Purple Coin mission on the Toy Time Galaxy where it replaces lava on the Luigi side.
Some Dark Matter exists in Bowser Jr.'s Boom Bunker. The same rule applies since the last game as being a hazard that instantly defeats Mario or Luigi.