The wide open sky; the heavens or firmament — often used poetically to refer to the blue expanse above.
飛行機が大空をゆっくりと進んでいった。
The airplane proceeded slowly across the wide sky.
子どもたちは大空の下で元気に走り回っていた。
The children were running around energetically under the wide sky.
晴れた日の大空はなんとも清々しい。
The clear sky on a sunny day feels so refreshing.