The “Museum Dimension”, also referred to as the “Primeval Dimension”, is a rumored Dimension where Arceus lives, and which hosts an early “snapshot” of the state of the Universe where Arceus and other Legendary Pokémon keep specimens of any and all Pokémon that have existed, exist or will exist, both as a form of testing ground and for archival and care for the history of their creation.

The dimension was rumored to exist and be reachable only by using the language of the Unown and the Azure Flute in order to sing a specific call to Arceus; because of this, the emergence of the Azure Flute in Sinnoh ca. 3730 was met with strong and definitive action by a roaming Articuno, who fled to the region to seize control over it, recover the Azure Flute and find a safe place where it could be disposed of ( “Suocéverse Legends” N/A? ).

Whether the dimension actually exists or what lies in there is unknown in-universe and undetermined out. The person closest to find out the answer ever was Cyrus in 3736 when he coerced the creator dragons Palkia and Dialga into creating an opening “to where Arceus resides” for him. Cyrus's whereabout and whether he was able to create his new world is unknown.

Were the dimension to exist, there would exist a number of portals or passages that would connect it to other, more earthly, dimensions and worlds in the Suocéverse. Among such would be counted the Entralink and Mt. Coronet in the The Present Era, the Spiral to Origin in the far future PMD universe, the post of the Lake Trio in Ancient History, an access in the Cave of Challenges, the bottom of the Sea Wall and the source point of the Dream World. The Azure Flute would be rumored to have the power of opening these access, as well as creating others temporarily.