Anti-matter

Anti-matter is a rare substance, the opposite of matter, having an explosive reaction when it comes into contact with its opposite. An example would be when plastic and anti-plastic meet (although it is not certain whether anti-plastic is made of anti-matter). (DW: Rose)

Omega was once stuck in a world of antimatter (inside a black hole), from there 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)

Following this attempt Omega tried once more this time using the Arc of Infinity to escape building himself a new body based on the Doctor's fifth incarnation. The Doctor however was forced to kill Omega as he was reverting to antimatter (on Earth). (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 always considered to be a theoretical concept, however, it could be simulated using computers, allowing Humans to power their freighters with it in the 26th century. An antimatter vessel, however, can only be stable if the ship itself is molecularly stable, otherwise it would explode. (DW: Earthshock)