Where are gazebo .world files ?

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I installed ros melodic desktop full version. Os used Ubuntu Bionic. Gazebo that got installed along with ros is 9.13.0. Now I am trying to follow http://wiki.ros.org/simulator_gazebo/Tutorials/StartingGazebo . When I execute roslaunch gazebo_ros empty_world.launch , gazebo is launched but no world view(the one with sky is there) . Observations:

1. Upon examining the gazebo_ros pack I do not find any .world files.

2. the empty_world.launch refers to

   <arg name="world_name" default="worlds/empty.world"/> <!-- Note: the world_name is with respect to GAZEBO_RESOURCE_PATH environmental variable -->
   

   but there is no .world file in the entire package.

3. Also the command env | grep GAZEBO_RESOURCE_PATH does not yield anything. The gazebo tutorial http://gazebosim.org/tutorials?tut=ros_roslaunch&cat=connect_ros says that .world files would be present at /worlds directory of Gazebo resource path but there is no gazebo resouce path.

Thank you.