Cygnus Alpha: When Jenna, experimenting with Liberator's controls, sent the ship hurtling forward at high speed, Avon mentioned "negative hyperspace", to which Blake responded: "Do you mean we crossed the antimatter interface?".
Pressure Point: Avon envisaged "antimatter screening" being built into the structure of the blockhouse entrance to Central Control.
Star One: Blake described the thousands of objects discovered by Avon out beyond Star One as "the biggest antimatter minefield ever put together". Since matter and anti-matter annihilate each other on contact, it may be that the mines were anti-matter generators rather than made of/containing anti-matter as such. The mine-field also acted as an early warning system, (probably relayed through Star One).
Dawn of the Gods: the theoretical negative universe on the other side of a black hole was reputed to be made of antimatter.