Un-removable "sidebar" in Theme

For starters, you can view my site at https://www.ImperiumUO.com

I seem to be having an issue with some sort of invisible “sidebar” that is preventing my background image from covering the entire screen.

The image is just fine on the left side of the screen, but on the right.. it’s crumpled together and the default grey background is visible. I’ve tried resizing the background image several times to no avail.

I should mention I’m currently using the Mantra theme, and despite dozens of searches, I’ve been unable to find a solution. Not quite sure my thread title is appropriate, considering I’ve never seen anything like this before – thus I haven’t any clue how to really describe it.

Any help would be immensely appreciated!