If you zoom in/out to the point that the window.innerWidth is 1925px or narrower, or body.offsetWidth is 1900px or narrower, there should not be any white background (more or less, this is difficult to compute on a laptop screen). However, if the body width exceeds those dimensions, a gap appears.
This seems related to using background-size:cover, which enlarges the background image proportionally, meaning the white center column also is enlarged. Can you edit the background image so there is enough photo in the center to compensate for wider displays?
Note: Firefox recomputes these dimensions when you see. Example: http://dev.jeffersonscher.com/resolution.html