Outer darkness is only referred to three times in scripture, but by using context we can establish what and where it is. The easiest thing to take for context is the gnashing of teeth, which occurs nine times in the Bible.
Outer Darkness
Pure Gold
The streets of heaven are paved with gold so pure that it's transparent. It would seem that this isn't the same kind of gold that we have on Earth, but another element that's not part of the known periodic table.