Earth prime

The core of reality
The centre of the mundane half of the cosmos, Earth Prime is (apart from perhaps the Conceptual Worlds), the most real place in the cosmos, metaphysically speaking. This is the dimension the rest of the multiverse revolves around.

Technically, the name is humanocentric. The milky way has multiple sentient species, and as such some prefer the use of "dimension Prime" or just "prime". However, whatever name is used, this world is the core of reality. As such, omega has tried to destroy it multiple times. If he succeeds, the multiverse may well fall with it.

(side note- despite the best efforts of extreme-range scryers and teleporters, no life has ever been found outside the milky way on earth prime, despite the milky way's thousands of species. Some think this is just a case of not having found it yet. Others have created more conspiracy-minded theories)