Anti-matter

Anti-matter was a rarely-found substance that was the opposite of ordinary matter.

Nature
Nothing could stop anti-matter particles, meaning that an anti-matter beam fired from a rifle at the ground would continue to spread into the ground infinitely. (AUDIO: Seven to One) Anti-matter was a kind of alter-time state, (PROSE: Going Once, Going Twice) being antithetical to the Time Lords' wat of interacting with the universe. (TV: The Three Doctors)

Anti-matter provided anti-quark energy. In collision with matter, it caused radiation annihilation, described by the Fourth Doctor as "a release of energy more powerful than nuclear fission." (TV: Planet of Evil)

History
After falling through the crack during the creation of the Eye of Harmony, Omega was stuck in a world of anti-matter, inside a black hole, from where he used his gell guards to attack UNIT HQ in an attempt to drag the Third Doctor to his home to take his place. (TV: The Three Doctors) He tried again to escape, using the Arc of Infinity, building himself a new body based on the Doctor's fifth incarnation. As Omega began reverting to anti-matter while on Earth, the Doctor was forced to kill him to prevent the cataclysmic explosion which would have resulted. (TV: Arc of Infinity)

Zeta Minor, the planet of the edge of the known universe, served as a gateway to an anti-matter dimension. The anti-matter creature attacked visitors to the planet and prevented them from taking its anti-matter minerals, prized as a power source. (TV: Planet of Evil)

The Osirian Court inoculated Apep with a battery of alter-time states, including anti-matter, in order to free him from the Great Houses' control. The final result was that Apep became "unanchored," a living patch of free reality identical to the creatures that existed before the anchoring of the thread. The process drove him insane, however, rendering him as useless to the Osirians as he now was to the Homeworlders. (PROSE: Going Once, Going Twice)

Among humans, as late as the 26th century, anti-matter was usually thought a theoretical concept. However, it could be simulated using computers, letting humans power their freighters with it even though they did not think it actually existed. An antimatter vessel could only be stable if the ship itself were molecularly stable — otherwise, it would explode. (TV: Earthshock)

However, an actual anti-matter drive powered a 67th century ship called Tsuranga. It used anti-matter particles to generate heat, which in turn generated the thrust required to move and power the ship. (TV: The Tsuranga Conundrum)

Incorrect references
On a ship of the Rulers of the Universe, River Song pretended there was a bomb charged with antimatter. (AUDIO: I Went to a Marvellous Party)

When a piece of a rag from a ghost was left behind accidentally, Panda incorrectly guessed that it was antimatter. (PROSE: Iris Wildthyme and the Unholy Ghost)