If this universe was designed, why is there so much lifeless “wasted space”? John Piper reflects on the reason for the mind-boggling size of the universe in A Peculiar Glory:
“The heavens declare the glory of God” (Ps. 19:1). This is why God made them - to put his majestic glory on display. The Hubble Space Telescope sends back infrared images of faint galaxies perhaps twelve billion light-years away (twelve billion times six trillion miles). Even within our Milky Way, there are stars so great as to defy description, like Eta Carinae, which is five million times brighter than our sun.
If you stumble over this vastness, thinking that it seems disproportionately large compared to the infinitesimally small man and his habitation, remember that the meaning of this magnitude is not mainly about us. It’s about God. “The heavens declare the glory of God.” The reason for “wasting” so much space on a universe to house a speck of humanity is to make a point about our Maker, not us. “Lift up your eyes on high and see: who created these? He who brings out their host by number, calling them all by name, by the greatness of his might, and because he is strong in power not one is missing” (Isa. 40:26).
God intends for the created world to communicate his majesty.
God knew when He created matter that its properties would necessitate a massive universe in order to support life (see here), so why create the kind of matter He created? To reveal His glory to us. That is the purpose of all creation, with all of it leading to the highest revelation of His glory: Jesus’ humble, loving, sacrificial death for us.