Gazebo Simulation files

1- if Gazebo showed a black-dimmed screen with a message like this:

This means Gazebo is currently downloading simulation models for you. If that took too long, you can alternatively download all Gazebo models offline and manually place then in the "models" folder (path: ~/.gazebo/models) as follows:

$ wget -l 2 -nc -r "http://models.gazebosim.org/"
$ cd models.gazebosim.org
$ mkdir -p ~/.gazebo/models/
$ cp -r * ~/.gazebo/models/