First off, I should tell you I'm not a parent, but a teacher, gifted individual and son of a gifted father. That said...while I was more thoughtful than emotional about them, I asked questions like this, and my dad's strategy was always to engage with me and wonder about the question himself, rather than trying to answer it. Maybe we would try to answer it together, maybe we would discuss possibilities, maybe we would come to the conclusion that it couldn't be answered - but just talking about it from the same perspective was a bonding experience. Gifted kids do look to their parents for security, but they grow out of expecting their parents to have all the answers pretty young.
Speaking as a teacher, by the way, I love kids who give me questions that don't have answers. It takes a special kind of person (at any age) to wonder about things like this.