Mew was the Original Being. Mew gave birth to the Egg of Arceus, and from there... Well,
Arceus did not create the universe on His own. His divine influence was passed on through His children to shape nature.
He took his essence and carved it in half to create Aether and Epoch who created stars and dimensions for His creations to exist in.
Arceus crowned daughter Aether in Pearl and named her Palkia. He then crowned son Epoch in Diamond and named him Dialga. Arceus, now aided by his children, created his final son Giratina from Dark Matter and the Void.
He formed the World with His breath and commanded His son Giratina to cradle it. Giratina would take the burden of His Father’s creation and nurture it, for it was His divine order.
Arceus combined the passion of Fire and the corporeal strength of Dragon to create Reshiram, the holy Angel of light whose fire burnt brighter than any star. As light could not exist without darkness, He then combined the raw power of Electricity and the corporeal strength of Dragon to create Zekrom, the holy Angel of dark whose essence is that of life's own spark. They were His soldiers, unto which the order of the Universe as it was known was upheld.
Arceus gazed upon His creations, content to let life bloom forward now that the seed had been planted. And He fell into a deep slumber.