Anti-matter
From Tardis Wiki, the free Doctor Who reference
Anti-matter is a substance that is rare to find. It is the opposite of matter, having an explosive reaction when it comes into contact with its opposite, such as when plastic and anti-plastic meet(although it is not known for sure that anti-plastic is made of anti-matter).
Traces of anti-matter could be found under the surface of the planet Zeta Minor.(DW:Planet of Evil)