My art teacher told me to add details to the background space to give it an atmosphere in the negative space. Well, I have no idea what to draw for details in the background space. What should I draw in the background space?

Also, the 1st picture attached is my teacher's example. The details that she added were clouds.
The second picture is my example, it doesn't have any details in the background space yet. (sorry if the quality of the picture is bad)