I'm not physicist too. In fact, it is because that I'm not a professional physicist that I could see the true nature of "void" as Matter in Motion. Most modern physicists are kind of Idealists, who believe in the existence of some "God particle" or "Theory of everything".
You're right that it's not enough to say "we just declare everything real to be real, checkmate idealists". In fact, real is not enough. The keypoint to differentiate Materialist and Idealist is that the Idealist suppose the existence of something in the world that untouchable, unchangeable, cannot be created, cannot be destroyed, but however can decide or influence everything in the world. Materialist don't think so, everything is mere mortal, it must be born from something and it also must die someday. If A can influence B, then B can influence A too. For example, have you ever thought about the nature of gravity? Why two bodies attract each other? Modern theory says that because of bodies will "wrap" the "spacetime" around themselves, however, why? By what mechanism bodies could wrap "spacetime"? And why the trajectory of a body must follow the curvature of "spacetime"? If a body is strong enough, can it resist "spacetime" and follow its own way?