In the beginning Arceus created Mew. Then from Mew he created Giratina, Palkia, Dialga, Groudon, Kyogre, Rayquazza, and all the other Legendaries. That's what we know. That's the ONLY thing us humans know...
Us humans believe that after creating the Legendaries, Arceus and his fellow Legendaries created the Universe. But no, despite Arceus' plan to create the Universe, he wasn't even close to THAT part of the plan...
You see, when Arceus created his Legendaries, they were just little kids...