Why should the nations say,
“Where is their God?”
Our God is in the heavens;
he does all that he pleases.
The focus of Psalm 115 is the clear superiority of Yahweh over worthless idol worship. The psalm cleverly contrasts the power and might of God with the impotence of idols. God is actively sovereign in the heavens, doing all that He pleases in the universe. We may not see Him physically, but He is clearly evident. Idols, on the other hand, are merely the work of human craftsmanship. Fashioned to look like us, they can't even do meager human things like eat, drink, listen, or speak, let alone rule the universe as God does. In the end, idolatry turns false worshipers into fruitless people. They become like their idols (Psalm 115:8).
God cannot be seen, but His work is clear. So if people scoff with "where is your God?", rest assured He is clearly right over their shoulder actively making Himself known. He cannot be captured in artist paint, in carving, or stonework, but He shows Himself in the sovereign rule of His people and the entire universe. His people satisfy Him by worshiping Him through knowing His position and power and not being merely content to sell trinkets of Him.
O unseen God of the universe,
I worship You. You are known, not by gilded statue in a temple, but by Your active Word and Spirit residing in my heart. You are shown, not in a glittering spectacle of stone made by people to decay, but in the sum of my days and the echoing thunder of Your Word throughout eternity. And You are powerful and mighty to save.