Ever's replies;
"Whatever, I don't care, it's not relevant to worlds being universes and was an aside to my main point.
"A massive core of energy lay beyond the door sought by the Heartless. It may be the ultimate goal of the Heartless. But what is that energy? I have devised a hypothesis, based upon my observations of the Heartless. The Heartless feed on other's hearts, and they yearn for that energy core. That thing beyond the door must be a heart, too--the heart of this world. There is no proof, but, having felt that immense energy, I am certain. That was the heart of the world. The Heartless are trying to take hearts not only from all living creatures, but from the planet itself. But what do they mean to do with the heart of the world?" -Ansem's Report 6
"The worlds Sora travels are divided into three regions: the realm of light, the realm of darkness, and the realm between. Worlds in the realm of light and the realm between exist as planets in a great sea of stars, which Sora and his friends explore aboard their smile-powered gummi ship. The realm of darkness, however, exists in a space closed off from time. It is accessible only via dark corridors—pathways that open for agents of darkness, or when worlds become unstable—or by using the "power of waking." As a result, Aqua has spent years wandering the realm of darkness, unable to return to the light." -Kingdom Hearts III's Glossary entry on "The Three Realms"
Tell me, how can worlds be universes when the series says they're planets on multiple occasions, both from the beginning of the series and the most recent installment. It makes the franchise's intent on defining worlds very clear. Worlds having different flows of time doesn't prove they're universes, nor does them being walled off from each other, it just means the verse's cosmology is very weird."