Manual:$wgNamespacesToBeSearchedDefault

Details
The variable holds an array of namespaces, indicating which namespaces are enabled for searching by default.

The array is indexed by the namespace constant.

Namespaces not added will not be searched by default.

Adding namespaces
To add other namespaces you just add this to your "LocalSettings.php" file, e. g. for the project namespace:

Changing options for existing users
In MediaWiki 1.16 and later, you do not have to change options for existing users.