Blackspace
Blackspace is an application that adds "blank" spaces (or "spacers") to the macOS Dock.
This is most commonly used as a way to place apps into logical categories.
To remove a space drag it off the Dock, like you would any other icon.
Download
How does it work?
It uses a programmatic equivalent to the following commands to add Dock spacers, so if you're comfortable with using the Terminal then just do the following instead:
> defaults write com.apple.dock persistent-apps -array-add '{"tile-type"="spacer-tile";}'
> killall Dock
Or, if you want a thin spacer, replaces spacer-tile by small-spacer-tile.
Enjoy.