In physics as with the other sciences, it is common and necessary to propose hypotheses to explain observed phenomena. In retrospect, some ideas seem laughable. They teach us to be cautious about accepting theories blindly. However, without imagination, it would not be possible to understand the underpinnings of the universe.
Phlogiston
In the 17th century the "science" of alchemy (the lead into gold business) had reached it's zenith. Alchemist Johann Becher was studying how and why things burned. He thought that when materials burned they released an invisible, odorless material that was later called "phlogiston". His research started a race to find out more about what exactly phlogiston was.
Many scientists conducted experiments to determine the properties of phlogiston but things just didn't add up. If phlogiston was released, why did some materials gain weight instead of lose it?
As it turns out, phlogiston wasn't anything but the absence of oxygen. When things burn (or rust), they oxidize. The material combines with oxygen, often releasing an enormous amount of energy. Sometimes the oxygen stays with the material, and sometimes the material combines with the oxygen and is released.
Phlogiston lead directly to the birth of chemistry. Oxygen was one of the first chemicals to be studied. The four element system of fire, water, air, and earth that had been developed by the ancient Greeks could no longer be supported as a viable theory.
Aether
The concept of nothingness is extremely difficult for the human mind to wrap around. We constantly want to fill in the void with something, anything. The concept of zero didn't gain widespread adoption until only about a thousand years ago. As it is with math, physics has it's own zero - vacuum, the nothing that makes space so spacious.
In the 18th century, Isaac Newton was studying light. Newton was sure that light was made of particles but that they had some properties of waves. From his perspective, waves required a medium to travel in, so he named this medium the aether.
For lack of a better explanation, this stood until the 20th century and which saw the emergence of relativity and quantum mechanics. Both these strange but accurate theories worked together to explain the observed properties of light, without the need for aether.
Aether continues to pop up now and again to help prop up one theory or another, but it has never been observed and is unlikely to exist.
