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.

The Time Lords once foresaw a time stream (PROSE: Doctor Who and the Genesis of the Daleks) or timeline (PROSE: Dalek Combat Training Manual) in which the Daleks would succeed in destroying all other life and so become the dominant species in the universe. The Fourth Doctor was sent to Skaro to interfere with the creation of the Daleks by Davros' hand, averting this time by creating the Genesis timeline by setting the Daleks back by 1000 years. (TV: Genesis of the Daleks) While some accounts indicate this averted timeline was the Dalek history which the Doctor had heretofore known, (PROSE: A Device of Death, Lungbarrow) others indicated it was an aberrance from the unchanging Dalek timeline which had been reinstated by the 1000-year delay. (PROSE: The Dalek Problem, A History of the Universe)

Nature
When Davros interrogated the Fourth Doctor on future Dalek conflicts, the Doctor mentioned that the Dalek invasion of Earth occurred in 2000. (TV: Genesis of the Daleks) By one account, this was evidence of a crucial divergence in history which allowed Daleks to come to power earlier, allowing them to one day destroy all life. (PROSE: A History of the Universe)

Extermination of all other life
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)