Your mod increases the number of chunks, but how to do the opposite?
I wanted to create an empty dimension (galaxy space) without chunks. 3D space and nothing more.

That would be 3D empty though. Create a void world, then the only way to place blocks is using the /setblock command (and related commands).

I was thinking about creating a space mod and I would like to create space as a separate dimension.

I need to understand how to do this at the code level.

As already suggested, you should make a void dimension, then you can add asteroids or whatever to that

If you have "no chunks" that means: blocks, mobs, players all can't exist in the world.