There are five layers to existence, to reality, to everything: Over, Aether, Ether, Nether, and Ender.
Over is both the simplest to explain and by far the most complex, because the Overworld is the universe as we know it (or as you know it; that is probably the better way to say it). It is planets and suns and distant stars, all separated by the cold, empty void of space. The closer you get to the Core, where magic once reigned, the odder things get. There is a higher concentration of planets made of floating islands, of spaces where the void is breathable, as if it were air by the sea.
If you get close enough to the Core, to the center of this galaxy, you find that planets themselves seem to cease to exist in favor of great rings drifting through space, grand floating islands hovering above nothing, and oceans without land beneath them. You step slightly further and now the air is breathable - the void, I mean. It is as if the chaos at the heart of the galaxy is simply a world as a server to itself, something created in chaos where order can reign, where life can survive, and death becomes something significant in its rarity.
The further from the Core you travel, out to the edges of the galaxy, the more you find something like an old sci-fi interpretation of planets and space. The further you travel out towards the edge, the fewer lingering remains of magic exist. Everything holds itself under its own power. Everything tends towards chaos with nothing to slow it down from its natural pursuit. Some worlds - some planets, called "servers" mostly these days - are entirely one biome, being too close or too far from their star for water and environments to truly thrive in their differences.