I did not google this because I assume someone here will put me to rights anyway, but I have it my head that there is considered to be a relationship between the value of 'c' and the (equally constant) amount of mass/energy (matter) in our universe. Atomicity is about the unit, ie, the possibility of there being multiples of one; this is I think also the basis of logic (the potential equivalence of two or more things, mental/conceptual or physical/real)*. But that does not apply to the (mental concept of) "matter"; we refer to matter in various forms but not various "matters" plural; although the meaningfulness of that may be a semantic illusion. I've equated matter with existence because it seems they are actually semantic equivalents. That means using the term matter in a more inclusive way than traditional physics, because eg, a photon particle is not considered a form of (atomic) matter, it's a form of (atomic) energy. But notice while it might be meaningful to refer to matter independent of energy, I am dubious about referring to energy independent of matter. Again, it seems that matter is a medium for energy / energy is a conceptual way of understanding matter (hence, it is not more matter, it is about relationships involving matter, so we say: energy is not matter, matter is not energy, but the two are intrinsicly intertwinned).
To my mind the idea that "existence" is just a semantic illusion seems logically implausible; but (logically) I don't see how it can be considered as anything other than matter (the inclusive "matter/energy" definition), which seems purely logical in nature (hence, the characteristics of the atomic form).
* there was a page I found in a GD thread a while back that involved a sort of semantic map of wikipedia, by way of which you could see how any given article related to another in a branching (historical/time axis) tree. A few people remarked how virtually everything was descended from math, but in fact, this was because almost everything required the concept of number and there are a few notable exceptions such as reason.