First, you have created that space yourself (by using absolute positioning).
Second, you shouldn’t do this, because the theme is responsive: it shrinks to adapt to lower screen resolutions and mobile devices. When this happens, the header image becomes smaller, so the positioning of the menu will be all wrong. Drag the browser window to make it narrower and see for yourself.
If you want the menu below the header image, the reasonable thing to do would be to use a theme that’s designed that way. If you insist on repositioning the menu on the theme you’re using, use a percentage instead of the px value.
Thanks justpi. I was provided with the CSS to create that space awhile back and I cannot recall what my issue was at that time. When you say “percentage” what would I use for that? Appreciate your help.