Dark Matter is blackish-purplish in colour, and comes in a liquid-gaseous state. It is normally contained in special containers, and used as a missile, or put into pellets and fired from a special gun.
Dark Matter is capable of turning anyone and anything it touches evil. For example, Cole, Kai, Zane, Jay and Nya were all turned evil by the substance, and made to serve the Overlord's bidding. Dark Matter has only one known antidote: to destroy the Overlord and the Garmatron. If they are destroyed, the effects of the Dark Matter will reverse, and everyone will revert to their pre-evil state.
Visible side effects
When contaminated by Dark Matter, the citizens of Ninjago City were turned into zombie-like beings, complete with rolled-back eyes and outstretched hands.
Dark Matter was likely first conceived by the Overlord, back during his imprisonment on the Dark Island. Later, the Garmatron was infused with Dark Matter technology, which would have allowed the Overlord to take over the world.
When Lord Garmadon, in an attempt to remake the world in his own image, activated the Celestial Clock, the Garmatron was unveiled, and given to Garmadon to use. Soon afterwards, Garmadon's body, however, was possesed by the Overlord. The Overlord then proceeded to shoot a missile full of Dark Matter to the mainland of Ninjago, corrupting some citizens. A short time later, after fighting the Golden Ninja, he created a portal to Ninjago, and drove through it. Once there, he proceeded to infect all of the citizens of Ninjago- all except for one. A short time later, the Overlord was defeated by Lloyd, and the Garmatron destroyed. All of the citizens reverted to their good sides, and the threat of Dark Matter was extinguished forever.