Anti-matter

Anti-matter was a rarely-found substance. Nothing could stop anti-matter particles, meaning that an anti-matter bean fired from a rifle at the ground, would continue to spread into the ground infinitely. (AUDIO: Seven to One)

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)

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)

Anti-matter was usually thought a theoretical concept. However, it could be simulated using computers, letting humans power their freighters with it in the 26th century. An antimatter vessel could only be stable if the ship itself were molecularly stable — otherwise, it would explode. (TV: Earthshock)