You can add a regular spacer or a small spacer. Copy and paste one of the scripts into your Terminal.
Regular spacer script:
defaults write com.apple.dock persistent-apps -array-add '{"tile-type"="spacer-tile";}'; killall Dock
Small spacer script:
defaults write com.apple.dock persistent-apps -array-add '{"tile-type"="small-spacer-tile";}'; killall Dock