What is The Expanse?

To the choirmaster. A Psalm of David. The heavens declare the glory of God, and the expanse proclaims his handwork. I recently ran across the ESVCE translation of Psalm 19, the beginning of which is above. I've taken the liberty of substituting the alternative translation of "the sky above," which the footnote says is equivalent in Hebrew to "the expanse", and invites comparison to Genesis 1:6-8