The Core Universe is the "Home" dimension that Humans occupied first. Although there are many universes, the Core Universe is called as such because of one reason, all other universes appear to all be similar to the Core Universe, but not to each other. Also, the Core Universe is the only dimension to actually hold intelligent life, which are Humans, and Humans alone.
The "farther" you get from the Core Universe, the similarities between other Bubble Universes decreases, and they are only linked to the Core Universe. Essentially, the Core Universe is the "tree trunk" of all the other universes that branch off it.
The Core Universe is the most developed of any of the dimensions in the Monoverse, but even then, Humans have yet to fully exhaust its exploration.