The Order of Mata Nui was a government that ruled more than half of the Galaxy between 100,000 BBY and 49,000 BBY, the other half being ruled by disputing Rakatan kingdoms between 100,000 BBY and 80,000 BBY , and then the League of the Six Kingdoms betweed 80,000 BBY and 50,000 BBY .
At the head of the Order of Mata Nui was Mata Nui himself, a celestial , who's life is thought to have exeeded 100 millenia. The rest of the order was composed of other celestials who had survived the first human rebellion, as well as the makuta, a race of shadowy beings created by the celestials in the distant past.
In 49'000 BBY, 1000 years after the Barraki rebellion , the Makuta rebelled against Mata Nui, killing him the members of the order. This coincided with the rebellion at Vitea , in which the human rebels fought a bitter battle in the Sandexi system , which ended when the planet Vitea itself destroyed the celestial fleet using its own inbuilt weapon, the Fire of Savoir.
By the end of 49,000 BBY, the celestials were extict and the Dominion of the Celestials fragmented.