I asked you to define space because your terminology is incredibly confusing.
distance spanned by space remains the same, as distance is to space what density is to matter.
I don't get the analogy; you can tell how much matter there is in a given volume if you know the density (or calculate the density given the mass and volume). The common (somewhat informal) definition of distance is "space between points".
and given that there is a difference between space and non-space
What is non-space, and how does it differ from space?
space is technically made of spacetime
This is either VERY wrong or you are redefining terms. Spacetime is an abstract concept; mathematically, it's a manifold with some metric. You are thinking of space as something material, which is not, and therefore it does not need to be composed of anything
this is advanced stuff, it's not surprising it's confusing.
distance is space between points. clearly, that depends on space. i like the analogy of a spring. you can get a pulled-out spring, and the length of the spring, along the material making it up, is set. compress the spring, it can fit in a smaller space that the stretched-out spring, but the length of it remains the same.
in that way, space can be thinner or thick. the density of the spring is constant each time, in the same way the time it takes to cross a certain amount of space remains the same. however, you can stretch out that space (like you stretch out the spring) so you can cover what seems to be, from an outside perspective, more distance, in the same time as it would take you to cross the compressed spring.
i am not thinking of space as a material, it's just very hard to explain it without relying on analogy.
space does have to be composed of some substance however, if it exists. scientific theory states space is expanding: what does that mean? if space is nothing, how could it expand? there is clearly a difference between space and non-space. non-space is what space expands into: non-space, essentially, doesn't exist (by any definition we could use).
space does exist: space is not nothing. if you disagree with either of those statements, please tell me why. if you do agree, as you should, then all i've said follows. if space is not nothing, then it is something.
that something is not matter, is higher-dimensional, but it clearly exists.