Why do Christians always use the fallacy of "something can't come from nothing"?

By that logic, don't you have to ask where god came from? a god can't come from nothing either by this logic. By the way, the notion that nothing can't come from something is somewhat of a paradigm in the context of what humans profess to know based on our current concept of cause and effect. Get it through your heads humans, if earth blew up tomorrow, the universe it would be like a 1/10 of one grain of sand from all the beaches and deserts on earth disappearing. not even a whole grain of sand...1/10 of one grain!