The Seriphia Galaxy was a galaxy that was a neighbour to Gallifrey's galaxy and was also designated as Galaxy 17A53.
It was four times the size of the Milky Way galaxy and home to over six hundred billion stars. The Dalek Empire detonated the Apocalypse Element within the Seriphean galaxy, causing widespread destruction. However, they accelerated time to ensure that the galaxy reformed, allowing the Daleks to establish a power base within it. (BFA: The Apocalypse Element)