Why does anything at all exist? William Lane Craig says,
This is the deepest question of philosophy: Why is there something rather than nothing? This mystery, which, according to Aristotle, lay at the very root of philosophy, is one which even thoughtful naturalists cannot avoid. [Atheist philosopher] Derek Parfit, for example, agrees, and I quote, “No question is more sublime than why there is a Universe: why there is anything rather than nothing.”
In the brief video below (excerpted from a longer lecture on “The Evidence for God”), Dr. Craig responds to this question, arguing that the universe requires an explanation (i.e., a reason for its existence), and the universe requires a cause.
If you need help introducing these ideas to your friends, the above video is a good way to get a conversation started. For additional simple (and sharable!) explanations of this topic, see these videos on the contingency argument and the cosmological argument.