Everything which exists, exists ultimately for God's glory. We see this principle taught, for instance, in Psalm 19:1, which states that the "heavens declare the glory of God, and the sky above proclaims his handiwork." Similarly, Psalm 8:3–4 indicates that one of the primary lessons of the sheer vastness of the cosmos is to demonstrate just how small and insignificant we human beings are: "When I look at your heavens, the work of your fingers, the moon and the stars, which you have set in place, what is man that you are mindful of him, and the son of man that you care for him?" The incredible size of the cosmos teaches us just how small we are, by showing us just how great God is.
If there is only life on earth, why did God create such a vast universe?
On the other hand, some people respond to these verses by saying, "Surely God didn't need to make the universe this big just to declare His glory, right? Even a universe ten times smaller than ours would still appear pretty impressive. Why did God need to give us so much space?"
Some non-Christians have even tried to use the (apparently) excessive size of the universe as evidence that maybe there really is no God after all. Surely if God did exist, they argue, He wouldn't have been so tremendously wasteful of space. Why create whole galaxies if no one is anywhere nearby to appreciate them? Surely an all-powerful and all-knowing God, if He existed, would have been smart enough to have given Himself an audience throughout the universe, if He were so insistent on making His glory known. A more plausible interpretation of the evidence—or so the skeptic's argument goes—is that there is no God at all, and that the universe is just big.
Unbelievers often take this reasoning one step further and ask, if God did not create life on earth, but it simply arose on its own, then surely elsewhere in the vast universe, life has arisen as well. After all, they say, there is nothing special about human beings; we are simply among the most highly evolved species on our planet. But what is to prevent us from thinking that life may have arisen elsewhere as well? There are so many planets, orbiting so many stars, in hundreds of billions of galaxies throughout the cosmos—surely, given all of this, we are not alone in the universe? The vastness of the universe, in the eyes of some, thus appears to simultaneously render the existence of God improbable, and the existence of life beyond the earth probable.
How should we as Christians respond to this? The discussion so far has made a few assumptions which are worth making explicit. First, we have assumed that God's glory must be declared for human beings specifically. But why should this be the case? Much of the universe was completely inaccessible to human observation for most of history—the concept of another galaxy outside our own was not even grasped until the 20th century. Does this mean that the universe failed to declare God's glory to people like David, who never saw a single galaxy as we do today? Of course not. If God's goal was to create galaxies to show human beings the greatness of His power, then this was completely unnecessary for most of human history. But what if this was not His only goal? What if God created the galaxies, not simply for the edification of mankind, but just because He likes making galaxies?
Interestingly, the notion of God's pleasure in His creation is suggested in Revelation 4:11, which states that "… you [God] created all things, and by your will they existed and were created." The Greek word for "will" in this verse can also be translated pleasure, wish, or desire. This verse seems to suggest that God takes great pleasure in seeing His own glory revealed through His creative power and intelligence, even when He is the only one to witness the creation.
The same principle applies to the possible existence of life beyond the universe: far from demonstrating that life is an unremarkable, ubiquitous, and naturalistically evolved phenomenon, discovering a new species of bacterial life in an ocean on Jupiter's moon Europa would be just as incredible a display of God's glory as discovering that new species in an ocean on Earth. The only reasonable conclusion is that God did not create the stars and galaxies throughout the cosmos or the many biological organisms in the world purely with mankind in mind; rather, God appears to have created a lot of galaxies because He likes creating them. If it should please God to place life at the very bottom of the Marianas Trench, where no one but He should ever even know it existed, why should He not also be delighted to place life on planets in the universe that we have no hope of visiting?
We are therefore in a position to address the original question as follows: even though there is no human being anywhere in the universe except on planet Earth, it does not follow that God's creativity and power are without an audience, even in the farthest reaches of distant galaxies. Instead, Scripture indicates that God actually takes delight in His creation, and He derives pleasure from the way in which His good creation declares His glory. God is therefore, in a sense, His own audience: He loves watching His creation do what He designed it to do. Whatever God has created—galaxies, solar systems, or planets with or without life (or both)—God has created for His own glory and pleasure in His creation. The vastness of the universe is therefore not an indication that God does not exist or that He is wasteful of space, but is rather an indication of God's unlimited creativity and power. And far from being a surprise, those who truly appreciate God's love for His creation might even expect Him to have placed new species of animals on distant planets which we have no hope of ever reaching.
In short, God's creation truly does declare His glory, and God Himself can delight in this, even if there are no human beings to appreciate His power and creativity. Whether the universe is large or small, and whether it is devoid of or teeming with life, God's creation declares His power, glory, and creativity.
Did God create the universe?
Why did God create humanity?
How do the laws of thermodynamics provide evidence for creationism?
What is the importance of biblical creationism?
Faith vs. science. Is there a contradiction between faith in God and science?
Truth about Creation