Discussing athiesm today, how do I address this?

[Nichomacheo]

As is normal for me, I sparked a discussion about religion at work today. Usually, theist's arguments are weak and easy to address. Today it veered off in a different direction, and I was completely confused as to how to respond to it.

My co-worker's question began as a simple "Where did the universe come from?" to which I responded, I don't know, no one knows for sure, but just because no one can answer it with certainty, it doesn't justify a divine creator. He then asks, "Well, with so much beauty and complexity in the world, are you saying that that came from nothing?" I reply, "The beauty that you see is merely the result of the interaction of mathematics and matter. Mathematics is the underlying language of the universe, and it makes things beautiful." Then he says, mathematics is only a human invention. Numbers do not exist without humans and that numbers are only our way of interpreting the universe. I asked him if 1+1=2 without us, and he says that the concept of "1" doesn't exist without us.

I had no idea what to say. The people listening looked at me like I had lost the argument. How do I respond? And, what's a better answer for the "how is there so much beauty in the world" question?