It’s caused by your dropdown menu – they are too long.
You really should fix it not for the white space’s sake, but the usability. A dropdown menu like you have now that require scrolling is the worst usable site one could experience, it may be just the only issue your site has, but just this alone, it can kill the rest of the effort you made for your site.
1. WIder the width of second level, so that each category item uses one line only, this may not solve the problem because you simply have too many categories in a parent category.
2. Make it to the so called mega-menu by making the width much wider, then float the LI to make two categories items stay in one line.
3. Re-work your categories structure.