The Wave of Light

Descartes thought light was a particle because he saw it moving in straight lines. But if light is a wave, then it opens this other question, what's doing the waving? And so you have to invent some stuff that we can't see, right? Dick Carr didn't know, so he just gave it a name. He called it Plenum. We don't know what it is, we just invent something to and give it a name as a placeholder.