Because it produces gravimotion that is gravitational pure dynamic-MOTION-volumes in its surrounding, the very mass of matter or the very mass of subatomic particles is occurring as pure dynamic-MOTION-volumes in gravimotion's interpretation of Nature.
In gravimotion's interpretation of Nature, the (so called) mass of any subatomic particle is occurring as concurring dynamic-MOTION-volumes toward a common point center of gravity, at the image of the gravimotion surrounding it.
Because the dynamic-MOTION-volumes implementing the present mo(ve)ment (blue in the ilustration) are the fastest and absolute, when two of them happen to butt heads on against each other (black in the illustration) they are forming an entangled pair; and such entangling mandates that pure-Motion-volumes have a front tip which can penetrate other pure-MOTION-volumes but are rebounding or stalling when the collision is occurring on their mutual front tips.
Occurring as 2 dynamic MOTION-volumes (momentum in science) in opposition, any such pair is now storing a high supply of constrained motion (energy in science), susceptible in reverse to be blasted away.
Within the present mo(ve)ment a number of these explosive-pairs may congregate together (only 3 pairs are shown in the illustration), all aggregated around a common center of gravity; they get stuck alltogether in such high density that they delineate the (so-called) mass of a subatomic particle.
Note that, in quantum mechanics, particles pop haphazardy out of empty space and are made of energy; in gravimotion's interpretation of Nature space-time is occurring as dynamic-MOTION-volumes out of which pop out particles occuring as gravitational MOTION.
A subatomic particle is occurring in such a high density of concurring dynamic-MOTION-volumes, that contrary to its surrounding gravimotion, it is occupying most of its allocated present mo(ve)ment volume.
The particle's (so called mass) very volume is determined by such a high density of aggregated dynamic-MOTION-volumes, that it can no longer be easily penetrated by other material particles of similar constitution.
In return, the demolition of such particle, that is reversing its contained yet explosive gravimotion, amounts to a blast of MOTION precisely described mathematically by Einstein's famous E=mC2 equation.
As such in gravimotion interpretation of Nature, " m " in this equation is a number with no unit, only defining a quantity, like " m = one hundred " or " m = one million " while light's MOTION squared " C2 " is implementing the active ingredient; and a great number "m" of "C2" does not implement a "quantity" moving faster than "C" as all "m's" are occurring in pairs of MOTIONS VOLUMES butting each other in opposite direction, maintaining them in an inert state motion wise; only an out of equilibrium implemented whithin each pairs of a material body, is setting the latter into motion; a loss of equilibrium within each of these pairs, is in effect implementing an over all MOTION lower than "C" that is lower than speed of light, unless totally released.
The squared C2 of light speed reflects (in this mind of mine) the entwining of the 2 dynamic-LIGHT-MOTION-volumes embodying the pairs.
One could imagine a subatomic particle constituted of trios rather than pairs; the three Motion volumes, of each trio, butting at each other and oriented at 240 degrees from each other. And that suggests that Einstein equation could also be written as E=mC3.
On how the present mo(ve)ment (space-time) produces material particles
The upper illustration shows how several pure-MOTION-volumes pairs, within the present mo(ve)ment, got aggregated together, yet without being transformed to a material particle, which amounts to an unfinished business necessitated to better describe the process.
The lower illustration shows the head-on colision of only two pure-MOTION-volumes within the present mo(ve)ment, and how their own MOTION-volumes are forming the volume of a subatomic particle.
In this case the material particle, which poped out of the present mo(ve)ment (space time in science) is characterized by 2 hemisphericalSPIN, as the 2 pure-MOTION-volumes, may among other, have transformed their colliding motion (energy in science) into two volumes of hemispherical spinning motions.
In this illustration the present mo(ve)ment's two pure-MOTION-volumes before colliding appear in blue, to indicate they are invisible to human eyes; on the other hand the material subatomic particle emerging out of the collision, whether opaque or translucid, because material is represented in black color.
Spinning tops mimick at our scale such subatomic material hemisphericalSPIN (elementary particle having mass in science).