[Mediawiki-l] How to change defaults for preferences for all users?

Hello fellow MediaWiki sysops,

Does anyone know where in http://www.MediaWiki.org/ - failing that, please share your experience here - we can find where and how to change defaults for user preferences so that subsequent users get the new defaults?

Is there a way to change defaults for preferences for even all users, including existing users? There are endless references to users changing their own preferences, but how to setup initial preferences?

For instance, in the user preferences under [Search] I want to engage ALL namespaces. I see the first is checked: [ x ] (main) ... but a search for "(main)" in the contents of all files in the mediawiki directory tree returns nothing! So where do we look to control these choices globally?

Thanks!