A hypothetical colorless, odorless, weightless substance once believed to be the combustible part of all flammable substances and to be given off as flame during burning. In the 18th century, Antoine Lavoisier proved that phlogiston does not exist.
During the 19th century, the idea was: if you couldn’t see it, it didn’t exist. Then radiation was discovered. And x-rays and radio waves and sonar. Although phlogiston actually doesn’t exist.