Dark Matter is a dangerous hazard that mainly appears in the Super Mario Galaxy games. They tend to eat through platforms causing Mario to jump over the holes these things create 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 and does not cause disintegration. Dark Matter appear similar to Poison Goo.
Dark Matter gather in dark blue colonies called patches, never being seen on their own. These patches vary in their size, though all of them have a network of purple lines that criss-cross the patch's surface, making them similar to Poison Goo. If his body touches the dark matter, Mario will be devoured alive and die instantly.
Dark Matter first appear in Super Mario Galaxy. Their only appearance is in Bowser's Dark Matter Plant. If he touches them, Mario/Luigi will instantly die as he is devoured alive. Poison Goo that resembles 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 exist in Bowser Jr.'s Boom Bunker. The same rule applies since the last game as being a hazard that instantly defeats Mario or Luigi. They also appear in the Cloudy Court Galaxy, lairing in the Purple Pond in the secret mission, Silver Stars in the Purple Pond.