How to set back screen to black when using a projector

Hi. I'm fairly new to the mac world. I've been using a mac for almost 5 years...but did none of the technical side until more recently.

We have a MacBook Pro connected to a projector. Everything works GREAT and it was incredibly simple to hook up.

The problem we are having is when a program is not open, the screen displayed on the wall is one of outerspace. I would like it to just be a black screen. When the computer was set up, this same picture is what was on the desktop as a background.

I tried changing the desktop which didn't work, and from there I don't know where to turn. I'm sure it's a simple easy fix that I just don't have the knowledge for...but that's why I turn to you guys!

Thanks in advance!

 

I changed the desktop picture. It didn't affect what was showing on the screen at all.

I'm not sure about the display. The only thing that is displayed on the projector is the open program. The main thing we use it for is powerpoint and it only shows on the screen when in slideshow mode.

The computer is not here right at this moment...I'll check that out! Thanks very much!