Galaxy

Universes such as N-Space contained millions of galaxies. (TV: Planet of the Ood) 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

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)

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

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

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)

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)

Other references

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

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)