When fluid has a certain concentration, it means a certain mass of the fluid is contained within a specific volume. Mass is the amount of matter. Volume is the amount of space in three dimensions. When space has concentrations, is the concentration analogous to concentrations of fluids? What is the mass? What is the volume?
It has no mass, and volume is an incomprehensible concept when applied to space. It's certainly an analogue, which is why I use the term. Imagining space as a series of coordinate points, and having those points be the equivalent of molecules in a fluid, is a very effective way to think of it.