The Nothing is a dimension mod that adds a void world that is parallel to the overworld. The Nothing is exactly as the name implies... nothing. But with the Nothing Anchor Block, you can use this place for many different things.
The Nothing all starts with one block, the Nothing Block:
In order to do anything with the Nothing block, you need to make a Nothing Punch:
You can use the punch to make Nothing Anchor Shells on a crafting table.
Now you can make the Nothing Machine:
With the Nothing Machine, you can create things like the Nothing Star, Nothing Key (used to travel between the overworld and The Nothing), and the Nothing Anchor Block:
The Nothing Anchor is a special block that works in all dimensions, but is especially useful in The Nothing. Right click with them in your hand and they will place themselves directly below you, making it possible to build where it would otherwise be impossible or saving you from a long fall by placing one before you fall too far.
The Nothing is free to be used in modpacks.