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 iin 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)

At one point antimatter was only a theoretical concept, however this didn't stop humans from powering their freighters with it. (DW: Earthshock)