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 1>Special:MyLanguage/Manual:Namespace constants|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:

Effect of adding/removing namespace
In MediaWiki 1.16 and later, adding or removing namespace will apply to all users.

In MediaWiki 1.15 and prior, adding or removing namespace will only affects anonymous users and accounts created after the change.