Anti-matter was a rarely-found substance. It produced an explosion on contact with its opposite, "normal" matter.
Omega was stuck in a world of anti-matter, inside a black hole, from where he used his gell guards to attack UNIT HQ in an attempt to drag the Third Doctor to his home to take his place. (TV: 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. As Omega began reverting to anti-matter while on Earth, the Doctor was forced to kill him to prevent the cataclysmic explosion which would have resulted. (TV: Arc of Infinity)
Anti-matter 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. (TV: Earthshock)