While it might come off defensive, it might be better just to raise it to the other's attention that they don't know either. It's Atheism 101 that theist answers aren't really answers at all, they just push the question up a level and declare it "ineffable". And I think a lot of theists would be willing to admit this. "God created the Universe. I don't know how God got created, but I'm willing to accept that I shouldn't expect to know this".

At this point, it's easy to point out that the atheist/naturalist position is at least as good. The atheist believes that there is some fact in the physical universe that he doesn't yet understand. The theist believes that there is some fact outside the physical universe that he can't understand. As long as the other doesn't expect you to know literally every fact about the Universe off the top of your head, you've defended your position to at least the level of his.

Of course, if you want to push the issue you can wheel in Occam's Razor to demonstrate that your position is vastly superior ("I don't know a fact" postulates a lot less than "There is a strange unknowable realm beyond all attempts at human enquiry"), but I think I'd just call it a day.

The stronger answer to many of those questions is "nobody knows."

Perhaps, but it would at best be a rethorical answer, and at worst an ignorant one.