- You may be looking for the eponymous organisation or Fifth Galaxy.
Galaxy Five fought a war with the Galactic Federation in the 40th century. Two of its agents were Eckersley and Azaxyr. (TV: The Monster of Peladon)
By some accounts, it was a physical galaxy. (PROSE: Doctor Who and the Monster of Peladon) It included the breeding grounds of the Dragonhunters (AUDIO: And You Will Obey Me) and may have been the same place as the Fifth Galaxy. (TV: The Daleks' Master Plan, et al.)
By another account, it was a terrorist organisation called Galaxy 5. (PROSE: Legacy) It was still a terrorist threat in 3999. (PROSE: Placebo Effect)
According to the Celestial Intervention Agency, Galaxy 5 was among those who allied with the Daleks in their ill-fated attack on Earth in 4000. (PROSE: A Sourcebook for Field Agents)