JustSpace adds what it is called, just space.

A new space dimension. To get there just travel real high.
Once there it's up to you how to travel and survive.
It could be as easy as to use an Elytra and Water breathing potions. Or you can add more mods like Create, Valkyrien Skies or Immersive Aircraft to travel the stars.

The new space dimension contains several new challenges and opportunities.
Just like our space there is no air in Minecraft's space. However there is some strange substance called Aether. It is not breathable. But it will make it possible to propel yourself with similar engines as in the sky, and the sun causes sun winds in the Aether making it possible to harness the power of wind even in space.

The space is naturally full of asteroids to mine for valuable resources. But there are also other mysteries in the vast space like abandoned ships and space stations and strange alien artifacts to explore. But what has happened to those once inhabiting those structures?

Finally space is also a way to transport yourself to new planets, or dimension as the superstitious tend to call them.

Basic FAQ

Q: How do I add a new dimension as a planet
A: You do it by changing the justspace-common.toml. Add the id of the dimension the the first list. The Name you want to give it to the second. Then the x and z position to the next two. Try too keep them at least 1000 blocks apart. Finally you need a texture that will be drawn in space when you are near or above the planet. See example below

#A list of items to log on common setup.
space_dimensions = ["minecraft:overworld", "minecraft:the_end"]
#A list of items to log on common setup.
planet_names = ["Earth", "Moon"]
#A list of items to log on common setup.
planets_x_pos = [0, 0]
#A list of items to log on common setup.
planets_z_pos = [0, 4000]
#A list of items to log on common setup.
planets_texture = ["textures/block/packed_ice.png", "textures/block/end_stone.png"]