Before the atomic concept was understood, Aristotle had convinced the Greek philosophers that only 5 elements were needed in order to explain how the world works:
air, water, fire, earth, with the fifth element, aether, was what we call "the fabric of space" (it did not exist on Earth, only "in the heavens").
The heaviest of the four elements was "earth", the next one was water, then came air and finally, the lightest of the four "physical" elements: fire. Aristotle saw any substance as a mix of these four elements, with the specific ratio being the explanation for the concept we now call density.
This is the reason why it was not accepted (until around 1687) that the Sun -- made of fire, the lightest element - could be the centre of the universe, since "Earth" (mostly made of earth) would have been the heaviest.
Aether was added because the Greek philosophers did not believe in "action at a distance". In order to explain how the heat and light from the Sun made it all the way to Earth (they knew that the Sun was much further away than the Moon), they had to imagine a medium that made up what we call "space".
Today, we understand the photon (the packet of energy that carries light and heat) as a self-transporting particle -- both particle and wave -- and we do not need the aether.
On the other hand, it seems we need the Higgs Field (which some could see as the new version of the Greek aether) as some kind of medium that explains inertia (therefore, gravity).