If u wanna space it away from the navbar, an empty pseudo element helps as very roughly outlined here WIP: 1.8 Mega Menu Not Staying Up if I Have to Reposition it - #2 by manc
Another (kinda hacky and dirty solution) is to use a gradient for the background of the submenu with the stops set so the first 10px or whatever are transparent,at that same spot the next color would start.