This question already has an answer here:
Looks like there was a keyboard shortcut for it. But in
10.11.6 all I get when I press
Cmd Shift . is the bonk sound.
Is that keyboard shortcut gone? What's an easy way to get it back?
Note that the shortcut works in file dialogs but not the standard Finder window.
Not a duplicate: the top ranked question on Hotkey to show hidden files and folders in File Open dialog? does not work in El Cap.