Anti-matter
From Tardis Wiki, the free Doctor Who reference
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)
Traces of anti-matter could be found under the surface of the planet Zeta Minor. (DW: Planet of Evil)
