system
Given the following post, respond as if you're commenting on Reddit:
human
How did we discover that space was a vacuum?

Who first proposed that space was a vacuum?

What did they base their hypothesis on?

How did we ultimately prove space was a vacuum?
gpt
"greek understanding of physics also progressed under Strato of Lampascus (c.335 - 269BCE.). He rejected the idea of a force pushing light objects, such as air, upwards to counter the force that pulls heavy objects down. He argues for the existence of a vacuum and showed that, because air can be compressed, voids must exist between the particles of which it is made up." - Source, a DK "Science Year by Year" Encyclopedia/book.
Later on it describes this "[Blaise] Pascal (1640's) also predicted that this pressure would diminish as higher altitudes. He asked his bother in law Florin Perier, who lived near a mountain, to test the idea. Receiving proof, Pascal suggested that air would thin out into a vacuum at still greater altitudes." 
It also says this. "for centuries, it was believed that air had no weight. But in fact it exerts a measurable force per surface area of the earth. Blaise Pascal demonstrated atmospheric pressure by inverting a mercury filled glass tube over a mercury reservoir. The tube's mercury falls to create an airless space (a vacuum), but atmospheric pressure pushes down on the reservoir to maintain a column: the bigger the pressure, the taller the column."

I hope this provides you with some of the insight you wanted.