Description: The ''plenum in the Scalar Theory of Everything (STOE) is the component of the universe that is a continuous medium that interacts with matter particles. Because a medium is not directly detected in instruments, the question of its reality is controversial. This is linked with the question of whether quantum waves are real waves in the plenum or merely probability waves. The experiment to demonstrate the reality of the plenum as a component of the universe has been done. The conclusion is that the plenum is a real component of the universe.