Since when do astronomers have conjectured that space is void, and not full of air like our immediate environment?
I am more interested about how long the notion has been around and how influential rather than when and how it has been scientifically demonstrated.
Background for the question: I heard a conference about philosophy of science explaining how Galileo revolutionized physics by postulating principles that where true, but defying immediate observation, like "all objects fall with the same acceleration" when in fact heavier ones tend to fall faster due to air resistance, or "undisturbed objects move in a straight line at constant speed" when such a trajectory can't be observed on Earth because of gravity and frictions.
I was wondering how he came to have these notions, then thought he might have been inspired by the motion of planets because he knew them very well and they move in a void so their movement is, as far as a XVI century astronomer is concerned, undisturbed.
But that would imply he knew space is actually void of air, which is not intuitive and requires observation technology he did not have access to (planes, weather balloon, sounding rockets...). So i was wondering since when this fact have been established or at least conjectured.