Known universe

According to Borusa, a President of Gallifrey would wield more power than anyone in the known universe upon their being introduced to the Matrix. (TV: The Invasion of Time)

The First Doctor referred to his TARDIS as "the most powerful space-time machine in the known universe." (TV: Twice Upon a Time)

The Fifth Doctor observed that Terminus was at the exact centre of the known universe. (TV: Terminus)

The Eleventh Doctor promoted the Church of the Papal Mainframe as the security hub of the known universe. (TV: The Time of the Doctor)

The Twelfth Doctor claimed that his TARDIS should have been able to detect anything in the known universe. This led him to determine that the Boneless were from another universe. (TV: Flatline)

Quantifer was located on the edge of the known universe. (TV: Resolution)

Behind the scenes
In NOTVALID: The Curse of Fatal Death, the Ninth Doctor calculates that he has saved every planet in the known universe a minimum number of 27 times.