Dalek timeline

"We foresee a time when they will have destroyed all other lifeforms and become the dominant creature in the universe."

- A Time Lord messenger.

As recorded in the Dalek Combat Training Manual, the Time Lords predicted a timeline in which the Daleks would succeed in destroying all other life and so become the dominant species in the universe. (PROSE: Dalek Combat Training Manual) It was for this reason that the Time Lords sent the Fourth Doctor to Skaro to interfere with the creation of the Daleks by Davros' hand. A Time Lord messenger was present to give the Doctor his mission and told him of the time that the Time Lords had foreseen, to which the Doctor admitted that it was "possible". (TV: Genesis of the Daleks)

Indeed, the first Daleks created by Davros acknowledged that their programming did "not permit to acknowledge that any creature [was] superior to the Daleks" and that "all inferior creatures were to be considered the enemy of the Daleks and destroyed". (TV: Genesis of the Daleks)

The Daleks came especially close to fulfilling this goal during the 21st century Dalek invasion when, with the support of Davros, they attempted to use the reality bomb to destroy all life across not only their universe but all universes save themselves, with the Supreme Dalek proclaiming that they would "become the only life forms in existence". Ultimately, however, this plan was thwarted by the DoctorDonna with the support of the Children of Time, with the Tenth Doctor crediting Donna Noble with the saving of the universe. (TV: Journey's End) In one universe hinging on a change to history resulting in Donna not being present for such an event, the stars began to go out; (TV: Turn Left) as well as people, the reality bomb was capable of destroying planets and stars. (TV: Journey's End) Siblings Different and Same visited timelines in which Donna herself had never existed. (PROSE: The Paradox Moon)