"Does everything have to be a duality like that? Can you have darkness without the concept of light?"
Can you have darkness without the concept of light? Simply put, no.
If you'd like another example, how about this... If there were no objects in the Universe, only free space, would gravity exist?
> "I believe in monism - ultimately there is only one thing, or one type of thing. It is not contrasted with anything else[1]. That's why it's not meaningful or meaningless, good or bad, right or wrong - it just "is."
[1] Except, perhaps, complete nothingness, utter void. So in that sense everything that exists is meaningful, and only pure nothingness is meaningless."
Have you considered there's another explanation that is the polar opposite to nihilism? Whilst it may not be what you thought of before, your comments above would suggest you'd be open to that possibility. To start exploring this...