Astral, from "ad astra" - "to the stars"

Aether, after the fifth classical element believed to fill the universe beyond Earth

The star sea.

The gulf between! The black! The astral expanse (not to be confused with my butt)! The final frontier! The celestial gates! The sucking void! The void of suck! Stellar blacktop! The velvet sea! The coldest hell!

Cosmos.

Firmament

DnD usually uses something like the Astral Plane. You could call it the Stellar Sea for the inky darkness of space.

I think you're looking for an adjective for the Material Plane though based on your description. Temporal vs Material.

I tend to use the term 'outer darkness' in reference to Matthew 8:12

The Heavens

One of my worlds calls it the Constellar Sea — the “Sea of Stars.” It’s described as a vast, primordial ocean where Naarin, the god of stories and starlight weaves constellations into being.

The constellations are memories, aye, but they’re also semi-sapient wards meant to protect existence from the Depths Beyond.

In truth, the Constellar Sea is like a fishbowl floating in dark, hungry waters. The wards keep the glass as strong as possible, but the pressure of the Depths - and the things which live in it - are always trying to shatter that barrier. 

I might be overthinking it but a bit more information would be helpful here. What does it mean to have a tether broken? This feels important to understand what is actually going on with the eternals which can help with the naming. For temporals, does the breaking of their tether to time free them from the bounds of time so they live outside linear time (like Dr Manhattan), or does it trap them in human linear time? Once we know we can apply this to space.

An eternal that’s FREED from the bounds of space, who can exist effectively everywhere at once could be called: Astral, Liminal, Universal, Spatial, or Dimensional. These would all pair well with Temporal bc they end with -al so they sound similar.

An eternal that is BOUND to our 3-dimensional space could be called Anchored or Fixed or Bound. There aren’t many good words here that end in -al.

Either way my recommendation would be to call them a Paron which is Greek for “present” as in ‘here.’ This could be a cool way of describing them as being always present because they are free from traditional space and so can be anywhere and everywhere or a cool way to describe them as stuck here, so they are only present “here.” (from Greek: https://en.wiktionary.org/wiki/%CF%80%CE%B1%CF%81%CF%8E%CE%BD). It doesn’t end in -al but it’s cool.

Temporal has two meanings: ‘relating to time’ or ‘relating to worldly, as distinct from spiritual affairs/secular?’ If you take the second definition You could also consider renaming your Temporal “Chronal” and then you could use Temporal for the space eternals.

A bit of clarity on which meaning you’re using and what the breaking of their tether does can help give you (and us) a lot of direction.

Ive been using “The Empty”

Wouldn't temporal mean the exact opposite of what you describe temporals to be? I'm very confused what's the logic here.

But, as for the question, I would have gone with ethereal, but you already use that to mean something else.

Cosmic might work. The literal counterpart to temporal would be material.

some terms used in my world

the eternal black, komose, the astral ocean, space, up beyond, star land

The Astrum

Untime
Void
Space
Aether
Niflheim
Cosmos
DER RAUM

Spatial would work, I guess. Or material. Maybe dynamic, since they have power over physical things?

I know that Sunless Skies refers to their version of space as "The High Wilderness"

Maybe look into the concept of Aether

Cosmos

The astral, or maybe the cosmic

Pathfinder calls the void between the stars The Black Tapestry.

I mean you could just refer to it by the scale of reference. Like solar system, Star cluster, Galactic arm, galaxy, etc

The Phlogiston. Taken from archaic english. Also, “Aether” (which is the exotic gasses that fill the Phlogiston). Friction current lights up the Aether with bow waves commonly seen by ground-dwellers as “Auroras”. Vessels riding the Aetheric currents also make a ripping or roaring sound as they pass through, which those on the ground experiencing the auroras can still faintly hear.

Phlogiston can be both singular or plural. Like the word “sheep”.

Therefore you have Aetherials and Phlogiston.

The Celestial Expanse

The Twinkling Sea

Space as in physical space? "Void" is a term classical philosophers used for space. Could also use "Ether", as used in the term "luminiferous ether", which is what old scientists called the medium of seemingly empty space that light travels through, before we realized that light doesn't travel through a medium.

Ginnungagap (gaping gap), which is the name of the cosmic abyss that existed before creation.

Atmospheren't.

What about Dimensional? I know time is often referred to as a dimension and so it could be seen as a category above the Temporal beings, but if youre trying to capture the vibe of the vastness they've untethered from it seems like its got potential

The Void Between

Akash

The cosmos, the heavens, the abyss. Basically anything that sounds kind of abstract and vague

The: Stars, heavens, galaxy, universe, void, expanse, ether, flow, gap, zero pressure, vacuum, nothing, emptiness, between, cosmos, range, stretch, astral,

The Expanse

Based on your edit you mean space as in time and space and not like outer space. With this in mind maybe these words will work:

Topological

Spatial

Dimensional

Volumetric

Positional or position

Locus

Locality

Expanse

Area

Extent

Interstice (I think this might be the one)

Lacuna

Void, the black, astral night, dark, deep dark, deep dark with twinkling lights, aether

Distance, emptiness, gap, void, 

Aether

I find naming things difficult:

As a world builder, I’m compelled to think about what the people my world call it.

As a writer, I know that jargon which distracts from familiar concepts can be harmful to readers.

With that said, the more you think about it, the more the term “space” kind of sucks. If you’re going to go for a new term, go for it! But beware of limiting other jargon.