Galaxy

According to the Tenth Doctor, universes such as N-Space contained millions, (TV: Planet of the Ood) potentially trillions of galaxies while constantly expanding. (PROSE: The Last Dodo) The Tiberian spiral galaxy, for one, contained a million star systems, a hundred million planets, and a billion trillion people when it was destroyed. (TV: Nightmare in Silver)

Galaxies

Identified galaxies

Named

Most of the Doctor's adventures took place in the galaxy known to the Time Lords as Mutter's Spiral, (TV: The Deadly Assassin) although he was also known to have visited other galaxies such as the Andromeda Galaxy and the Isop Galaxy. (PROSE: Alien Bodies, TV: Castrovalva, The Web Planet)

The Daleks recreated the Seriphia Galaxy with the Apocalypse Element and used it as their main base of operations during their invasions of Mutter's Spiral. (AUDIO: The Apocalypse Element, Dalek Empire)

Urlic was the last planet of the Catorkian Galaxy by 2050. (TV: Lost Library of Ukko)

The Tiberian spiral galaxy was a galaxy sacrificed by the human empire to destroy all Cybermen, bringing the Cyber-Wars to an end. The human emperor, Ludens Nimrod Kendrick Cord Longstaff XLI, ruled a thousand galaxies. (TV: Nightmare in Silver)

Whilst travelling with the Tenth Doctor, Donna Noble witnessed the destruction of the Mestophelix Galaxy. (COMIC: The Widow's Curse)

The Tenth Doctor named a galaxy Alison. (TV: The End of Time)

The Fraxis Galaxy contained the constellation of Zandrobar and the planet Zorth. (AUDIO: Tartarus)

The West Galaxy was the location of the Generios system. (AUDIO: The One Doctor)

The Wrarth Galaxy was a galaxy on the other side of the universe from Mutter's Spiral, inhabited by the Wrarth Warriors and the Meeps. (COMIC: Doctor Who and the Star Beast)

Galaxies designated as junk galaxies were home to junk planets. (TV: The Tsuranga Conundrum)

Numbered

Galaxy Three was a galaxy containing the planet Wrechon Four and the Ninexie homeworld. (AUDIO: Absolute Power)

The Third Galaxy was the galaxy in which the planets Urbinia and Baralda were located. (AUDIO: Daughter of the Gods)

The planet Drahva, home of the hierarchical, feminine-appearing Drahvins, was located in Galaxy 4. (TV: Galaxy 4)

The Fourth Galaxy was one of the Outer Galaxies. (TV: The Daleks' Master Plan)

The Dragonhunters had breeding grounds in Galaxy Five. (AUDIO: And You Will Obey Me)

The Fifth Galaxy was one of the Outer Galaxies, represented at the Galactic Council by Zephon, its Master in 4000. (TV: The Daleks' Master Plan)

Galaxy 7 was a galaxy that contained the planet Hedenex, home to the Hedenexians. (AUDIO: Benny's Story)

The Seventh Galaxy, also called the Sculptor Dwarf Galaxy, (PROSE: The Eyeless) was a part of the Seven Galaxies (TV: Genesis of the Daleks) located 290,000 light-years from Earth. (PROSE: The Eyeless) According to one account, it was the location of Skaro, the Dalek homeworld. (PROSE: The Infinity Doctors)

The Seventh Spiral Galaxy was the galaxy where the Koth-Kulaar originated. (COMIC: The Moon of Lost Hope)

According to one account, the Eighth Galaxy was the location of Skaro. (AUDIO: The Destroyers)

The Tenth Galaxy was the largest of the Outer Galaxies, represented by Trantis on the Galactic Council. (TV: The Daleks' Master Plan)

The Thirteenth Galaxy was a galaxy where the thirteenth planet was located. (COMIC: Triskaidekaphobia)

The 29th Galaxy was a galaxy where Kerun Za was located. (COMIC: Sea-Rah)

Galaxy 57 was the galaxy in which Usunru was located. (COMIC: The Cleverest King)

Galaxy 357 was a galaxy inhabited by the cloud hangers. (AUDIO: I Am The Master)

Unidentified galaxies

Eugene Jones supposedly "worked out" which galaxy the Dogon Sixth Eye hailed from. (TV: Random Shoes)

According to Jack Harkness, the space whale known as "Cash Cow" hailed from a galaxy beyond Mutter's Spiral. (TV: Meat)

Sarah Jane Smith observed that Erasmus Darkening hailed from a galaxy other than Mutter's Spiral. (TV: The Eternity Trap)

The Bank of Karabraxos was promoted as the greatest and most secure bank in "the galaxy". (TV: Time Heist)

Groups of galaxies

The Tri-Galactic was a trio of galaxies, including Mutter's Spiral, in which the Second Great and Bountiful Human Empire was active around the 42nd century. (TV: Planet of the Ood)

The Five Galaxies were a group of galaxies which included Mutter's Spiral. (PROSE: Borrowed Time, TV: Delta and the Bannermen)

The Seven Galaxies were a collection of galaxies near and at the centre of the universe. (TV: Twice Upon a Time) The Kaleds believed that Skaro, which existed in the Seventh Galaxy, (PROSE: The Infinity Doctors) was the only planet capable of supporting life in all of the Seven Galaxies. (TV: Genesis of the Daleks)

The Nine Galaxies were a group of galaxies. (PROSE: Fascination, AUDIO: The Dispossessed)

The Ten Galaxies were a group of galaxies ruled by the Dominators and their Quark servants, calling themselves the "Masters of the Ten Galaxies," (TV: The Dominators) although the claim went unchallenged (PROSE: Mutually Assured Domination) and William Bishop later wondered if it was simply propaganda. (PROSE: The Two Brigadiers) Among the galaxies were Phi-Ralix, the Dominators' home galaxy, the Mutter's Spiral (PROSE: Mutually Assured Domination) and, in the far future, the galaxy QSO 0046. (PROSE: Timewyrm: Apocalypse)

The Twelve Galaxies were a group of galaxies. (TV: Dragonfire, The Ghost Monument)

The Alignment of Exodor were seventeen galaxies in perfect unison. (TV: Closing Time)

In the year 4000, the Daleks held a meeting with representatives from seven Outer Galaxies to plot the invasion and overthrow of human-occupied space. By that time, they had already taken over at least seventy planets in the Ninth Galactic System. (TV: Mission to the Unknown)

Fate

In N-Space, the galaxies had collapsed by the end of the universe in the year 100,000,000,000,000. The Tenth Doctor told Professor Yana that, had he been in a different time, he would have been revered throughout the galaxies. (TV: Utopia)

Intergalactic

Organisations which operated intergalactically included the Intergalactic Flora Society, (PROSE: The Room With No Doors) the Intergalactic Floral Society, (TV: The Seeds of Doom) Intergalactic News (COMIC: Terrorformer) and the Intergalactic Union of Pan-Dimensional Assistants and Companions. (PROSE: Project: Wildthyme)

Intergalactic law spanned galaxies, (TV: Carnival of Monsters) as did the intergalactic rules of engagement, (TV: The Poison Sky) the Intergalactic Olympic Games, (PROSE: Placebo Effect) the Intergalactic Song Contest, (AUDIO: Bang-Bang-a-Boom!) the Intergalactic Staring Championship (PROSE: Ghosts of India) and the Intergalactic Treaty of Cultural Rights. (COMIC: Echo)

The Thirteenth Doctor identified the Judoon as intergalactic police and mercenaries. (TV: Fugitive of the Judoon)

Other references

In 1958, Lord Azlok boasted that the Viperox had laid waste to entire galaxies. (TV: Dreamland)

The Eleventh Doctor noted that Saborian smelling salts were outlawed "in seven galaxies", (TV: The Power of Three) while magno-grabs were outlawed in "most galaxies". (TV: Journey to the Centre of the TARDIS)

The Moment was known as the "Galaxy Eater". (TV: The Day of the Doctor)

Davros reflected that galaxies had burned in the time that he and the Doctor knew each other. (TV: The Witch's Familiar)