The Seven Galaxies were a collection of galaxies. 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)
They were used in a question by the Eighth Doctor, "Why in the Seven Galaxies..." (AUDIO: Neverland) and the expression "What in the Seven Galaxies?" by Romana II (AUDIO: Zagreus) and Heff. (AUDIO: Tomb Ship)
The planet Villengard's weapon factories were know as the "Nightmare of the Seven Galaxies" before it's destruction by the Ninth Doctor. (TV: Twice Upon a Time)