It appears that there is a lot of mass that’s missing. In fact, this used to be called the missing mass problem. So what’s going on here? One possible explanation is that there is a lot of matter—a lot of physical stuff—arranged in a “halo” (a spherical shape surrounding galaxies). This has become known as dark matter. It’s physical stuff, thus the word “matter”—but we don’t see it, thus the word “dark.” …