Where is the center of the universe? That's difficult to access, although not so in the DC Universe(s). When the Multiverse was first created from the rash actions of the Malthusian known as Krona, all planets in Earth-One's realm were replicated countless times throughout the cosmos except for Oa itself (on the right). That is, except in the Anti-Matter Universe which had the world of Qward (on the left), which was the home of the Guardians of the prime Universe's evil dopplegangers, the Weaponers. What resided in the remaining dimensional planes at their center? A mysterious monolith... the Rock of Eternity... created by Shazam in his Champion alias with the help of his time-lost pupil Captain Marvel. This locale, at the center of space and time, could not exist in the original matter or the anti-matter universes, but could be accessesd from Earth-S, and elsewhere.
The interesting aspect of the Rock is that it was an access point to Eternity, a timeless access point wherein historical and fictional characters reside. Kid Eternity's powers tied into this location wherein he was able to recruit various heroes, including Blackhawk. While that ace aviator had cosmic twins on Earths One and Two, on Earth-S his tales as well as those of his fellow Quality Comics allies were only published. And yet, these characters were real individuals on Earth-S thanks to being residents in the ether of Eternity.
Further evidence that something significant resided in the center of Earth-Two's universe is evident during the Infinite Crisis, when the Superboy of Earth-Prime was assigned the task by Earth-Three's Alex Luthor of recreating that lost realm by shifting the center of the surviving universe by moving various surrounding worlds' positions in space. The resulting mass exerted from the repositioned planets in close proximity created an access point in space-time, as had the Rock of Eternity.