While the devs have never been 100% clear on the parallel dimensions stuff and how it works (are there Jovian planets etc), the stuff they've said tends to lean towards different planes - dimensions "at a different frequency" as folks say.

That said, there is one piece of evidence that I never see mentioned in these threads, and which is even more stark in reference to Shadowbringers where so much emphasis is put on the night sky in one of these worlds: look up. There are stars there. What are those stars? There is at least an empyrean sphere containing our solar system, if not a whole galaxy or universe, because there has to be somewhere for the stars to be.

What might be a neat experiment would be to snap a screenshot looking at some common monument - say the crystal tower or Eulmore/Limsa - and see if the stars line up identically or are different. If they line up, then we're talking about duplicated solar systems/galaxies/universe. If not, then the "void beween worlds" might be space, though with very different rules.

Of course, this opens up lore implications of its own. If every shard is a universe comparable in size to our own, then there might be whole worlds of alien lifeforms consumed by the floods of light or darkness.