Nothing unusual about that, of course, but he raised a question that I must since have had quietly gestating in my brain until a better answer than the one I gave could hatch. Specifically: "how do you explain why there is something rather than nothing?"
The answer I gave was a rambling explanation of the Anthropic Principle and the fact that in a quantum universe, superimposing the states of both being and not-being is in no way impossible or paradoxical which, unsurprisingly, my opponent rejected as not making a lot of sense. As is usual when discussing physics, frankly. If you think you understand quantum physics, you don't. Our brains are just not equipped to intuitively handle the probabilistic weirdness that underpins all space, time, force, energy and matter.
But a few alternative answers have since occurred to me
The first is that the question itself may not necessarily be a valid one. It could be exactly as much of a nonsense question as "what is the marital status of the colour blue?" or "How much does Nostalgia weigh?" albeit better disguised. More like "why is Pi irrational? Why not an integer?" I suppose. On the surface it looks like a valid question, until you realise that it simply couldn't be anything other than what it is. There is no scenario in which the circumference of a circle is exactly three times its diameter, or indeed is even a rational multiple. It just isn't possible for Pi to be anything other than Pi. You can ask why we take the ratio between circumference and diameter as opposed to radius, but asking why the number is irrational is itself irrational and pointless.
The same response might apply to the universe. the question "Why is there something rather than nothing?" may simply be a well-camouflaged nonsense. It may be that, in the scenario that my above quantum superimposition of being and not-being is inaccurate, that the state of not-being is simply and fundamentally as impossible as a rational Pi.
The second and rather more damning alternative answer that occurred to me is quite simply that throwing God into the equation to make things up just shifts the question onto the creator, thereby adding a layer of complexity. Whence came God? What would be the reason for him to exist, as opposed to nothingness? Why should these questions stop the moment we step up a tier? Why is it permissible for God to auto-originate, but not the universe?
Failure to think things through, I guess. Which is why these lovely slow-brewed observations will almost certainly never actually manage to get through to one of these people.
Good thing I argue for the joy of it, rather than to change minds then.