Anti-matter

Anti-matter was a rarely-found substance. It produced an explosive reaction when it came into contact with its opposite, 'normal' matter.

Omega was stuck in a world of antimatter (inside a black hole), from where he used his gell guards to attack UNIT HQ in an attempt to drag the Doctor to his home to take his place. (DW: The Three Doctors)

Omega tried again to escape, using the Arc of Infinity. He built himself a new body based on the Doctor's fifth incarnation. The Doctor had to kill Omega, who was reverting to antimatter (on Earth), to prevent the cataclysmic explosion which would have resulted. (DW: Arc of Infinity)

Traces of anti-matter could be found under the surface of the planet Zeta Minor. (DW: Planet of Evil)

Antimatter 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. (DW: Earthshock)