Who proved that there is there is no zero gravity in the universe?
Answers
I would not be surprised if this was already suspected, at least, by some ancient Greek philosophers. But it was certainly well understood by Newton. Here is a famous illustration from his Principia:
It demonstrates how a projectile fired from a cannon horizontally, if its velocity is sufficient (and ignoring air resistance), would fail to impact the Earth as the Earth would curve away under it. So even though it is continuously falling, it never hits the ground. And that’s precisely what spacecraft and the Moon do around the Earth, or the Earth and other planets do around the Sun.
So to clarify once again this business about space and gravity: Space has gravity. What space lacks is a floor that holds you in place against the pull of gravity.