When the priests brought the ark into Solomon's temple, what filled the house of the Lord?

Answer: A cloud

The cloud filled the temple so the priests could not stand to minister, for the glory of the Lord filled it.

“And when the priests came out of the Holy Place, the cloud filled the house of the LORD so that the priests could not stand there to minister because of the cloud. For the glory of the LORD filled the house of the LORD.”— 1 Kings 8:10-11 (BSB)