God, the greatest "of all invisible realities" created the universe, the greatest "of all visible things." We know this by faith, which is not acquired through observation of the physical senses but through the orientation of a regenerate soul in communion with the Wisdom of God. "And this Wisdom, entering into holy souls, makes of them the friends and prophets of God and reveals to them, silently and interiorly, what God has done."
Some people struggle to believe that God actually created the universe, and hold instead that the universe must have always existed. Or, at the very least, God must have been eternally creating the universe. Otherwise (and assuming the doubter in question is a theist), we have to admit some kind of change in the unchangeable God. That is, there must have been a moment in eternity past when God had the inspiration to create, where before He did not have the idea. Which means that God changed in His knowledge and will.
Yet, such people often likewise hold the idea of an eternal soul. But we know from experience that the soul can have new emotions and new sensations, so clearly this does not negate the argument all together. (And of course, the human soul is not eternal in the sense that it always existed--only in the sense that it will always exist henceforth.)
As to those who "question us about the time at which it was created," that is, those who ask why God created the universe at the time He did, and not at some time in eternity (farther) past or eternity (farther) present? In response, we might ask why God created the universe in the space where He did. That is, why is the universe where it is now, and not some other place? The cosmos clearly has a place of its own and, barring Epicurus (whom Augustine dismisses with an argument that I hope to return to in a later blog post), all good philosophers admit that the cosmos is limited, "In that case, let this be the answer: It is silly for them to excogitate a past time during which God was unoccupied for the simple reason that there was no such thing as a time before the universe was made"--just as there is not a space beyond that of the space of the cosmos.