The X Continuum was an omnipresent, shared existence meant for a race of beings known as the X to be constructed and developed by the Omni and to rival the Q Continuum. (Star Trek: Phoenix-X)

History

The First X

At some point in history, a rogue Q escaped the Q Continuum and changed his name to X. This frustrated the Q, who then captured X and prisoned him in a small sphere within an anomalous nebula.

In 2376, the USS Phoenix-X unwittingly released X from this prison, prompting Q to return and confront. Instead of going back into his prison, X opted to end his own life and transfer his X powers to Daniel. ("The X Continuum")

Omni Grooming

In 2377, more Qs revolted and changed their names to X. One of the Xs visited Daniel and receded the majority of Daniel's given X powers in order to feed it into an iso-star and begin X Time. The other Xs dispersed their own powers amongst mortals throughout the Milky Way Galaxy to turn them into Omni and have them grow their own X power to one day construct and start the rival Continuum. ("Vengeance", "The Nega'Jem, Part II")

Feeding the deceased X's X power into the iso-star caused that part of space to turn bubble a chaotic temporal barrier around a growing pocket space called X Time. It was predestined to last at least a year and a half in existence. It was meant to eventually house all Omni, where they would come to finish their growth and start the Continuum. In this place, they would expedite their X power growth through fighting each other. It was said one could only enter X-Time at its creation or right at its end.

In 2378, during the end of X Time, Q himself came to retrieve the X and return them to the Q Continuum, including the original X power from the first X. The Omni kept the power they grew for themselves. Though once promised by the Q Continuum that these types of beings could not exist, it was ruled harmless for Omni to go on. As for the X Continuum, it remained a goal, that many Omni still believed in. ("Avalon Battlefield")

X Continuum

In 2383, Rein became the first Omni to complete his growth and ascend into full omnipotent X status, starting the X Continuum. ("Broken Pieces")

In 2384, Q confirmed the existence of the X Continuum when confronting Cell (previously known as Daniel) during a mission to the Andromeda Galaxy. ("Forbidden Crossing, Part II")