Reverse matter

Reverse matter was a type of matter in which atomic structures were reversed. It was not the same thing as anti-matter. Objects made of reverse matter could be detected by radio waves but were invisible to optical instruments. TARDISes had difficulty materialising in reverse matter environments.

In the 1970s, the Zerons, a species native to a reverse matter galaxy, came to Earth to convert it to reverse matter and claim it as their own, but were stopped by the Third Doctor. (COMIC: The Enemy from Nowhere)