BottomNavigationView is not full width on Android

The BottomNavigationView is not displayed as full width because it is not supposed to.
If you want to fill the space behind the BottomNavigationView, you can set the background color of the view to be the same color as the items background:

<android.support.design.widget.BottomNavigationView
     android:background="@color/bottom_view_color"
     app:itemBackground="@color/bottom_view_color"

     // .... />

References
http://stackoverflow.com/questions/41432902/bottomnavigationview-is-not-full-width/