Event One

Event One or the Big Bang refers to the creation of the present Universe itself and the beginning of all ages.

At least one universe existed before this one (NA: The Pit, MA: Millennial Rites, SJA: Secrets of the Stars). The Great Old Ones and The Beast existed and were able to survive the big bang (though the Great Old Ones gained advanced powers in the process) (PDA: The Quantum Archangel, DW: The Satan Pit). The energy entity known as the Ancient Lights continued some sort of existence after Event One, but was unable to enter that new universe (SJA: Secrets of the Stars).

That universe, in which Doctor Who took place from the First Doctor's adventures, through the end of Series 5, has been destroyed. At the end of Series 5, a new big bang was created by the Doctor, to replace the destruction of the old universe. (DW: The Big Bang)

During the first period, there was "empty void" (DW: Terminus). Matter then exploded outward in the form of hydrogen (DW: Castrovalva).

The Doctor speculated that the massive ship Terminus had time jumped at some point, discharged its fuel and instigated the Big Bang before time jumping a second time back from the distant past. (DW: Terminus)
 * These two descriptions may be of the same event; hydrogen is an excellent fuel for fusion, and may be the fuel dumped by Terminus. If Terminus did cause Event One, then the entire universe is a predestination paradox.

The Leptonic Era would follow. (DW: Time and the Rani)

Possibly Apocryphal Accounts
Contradicting this, a Time Lord from the High Council stated to the Doctor that the Vipod Mor, headed back in time to the creation of a galaxy, would overshoot and travel back to before the universe, thus causing the Big Bang as a "dense monoblock of matter.". (BBCR: Slipback)
 * These two reasons may both have been true, with the fuel of Terminus colliding with the Vipod Mor.

References to the Big Bang

 * A Time Lord told the Doctor that they (the Time Lords) had mastered transmat technology when "the universe was half its present size." (DW: Genesis of the Daleks')
 * The Time Lord said this to the Doctor on Skaro during an indefinite time period during the end of the Thousand Year War, which happened at, the very latest, before the year 2157, by which time the Daleks already had interstellar spaceflight.


 * According to the Doctor, the creation of the universe occured 13,500,020,012 years before a specific date in the early 22nd century, when he decided to celebrate. (NA: Transit)
 * We do not know if he counted the date in Earth or Gallifreyan years.


 * The Doctor implied that he had watched it happen, or at least that other sentient beings had done so. (DW: Destiny of the Daleks)
 * The delusional Monarch, believing himself God, wished his ship to travel back to the Big Bang and witness himself creating the universe. (DW: Four to Doomsday)