My desktop is not filling the entire monitor screen.

It looks like some glitch may have confused the OS.

If a simple reboot of your Mac doesn’t fix things, see if the problem still happens in Safe mode (press and hold the shift key down at startup). It can take much longer to safe boot (5 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode is non-destructive to user data. It’s not a cure-all, but very often corrects weird software behavior by forcing the OS to do cache clearing and other housekeeping. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

Does the problem persist while in Safe mode?

Often not required, but consider allowing the Mac to "soak" in Safe mode for a couple hours to allow maintenance routines and processes time to complete, then exit safe mode by restarting your Mac normally and re-evaluate the issue again.

Double click on an empty space on the menu bar.

I'm not sure what you're referring to when you say "background", as the screenshot you've provided shows the background covering the entire screen.

Are you referring to apps that aren't filling the entire screen, or something else?