Materialism is the view that everything that exists is made of matter. What’s matter? It’s hard to say with both precision and completeness, but it can’t be far off to think of matter as whatever can engage causally with the known forces of nature: gravity, electromagnetism, and atomic forces (strong and weak). If a thing responds to any of those forces, that thing is material. Of course, maybe there are some unknown forces of nature, and we’ll have to revise as they become known, but right now, this seems to be an adequate criterion for judging what counts as matter.
But I don’t think materialism is true, and it’s not because I believe in spirits or love or imagination or magic. It’s because of math. Math is a science of form: it explores the possible forms or properties or systems that are possible. Some of these possible structures, of course, describe the real systems we come across in our world, which is neat, and makes physics possible. But there are many, many more possibilities than are actual. It doesn’t take many beers before a gang of interested mathematicians will start describing all sorts of things that could never come to exist in our puny world because they are too big or complicated.