One Space Exists and has the Properties of a Wave Medium
If One thing, Space, Exists, how can a second thing exist within the One thing Space that gives rise to Matter and its Motion?
As Aristotle importantly and profoundly says;
... there is some other cause of the change. And to seek for this is to seek for the second kind of principle, as we would say, that from which comes the beginning of the change. (Aristotle, Metaphysics)
Unless the further factor is active, there will still be no movement. (Aristotle, Metaphysics)
There must then be a principle of such a kind that its substance is activity. (Aristotle, Metaphysics)
I feel quite sure that if Aristotle had known of Huygens' work on Wave-Motion, along with the work of Newton and Leibniz, (who all lived 2,200 years later in human history) then Aristotle would have solved the problem of how Matter exists in this Space of our Universe. Unfortunately no formal knowledge of Wave-Motion existed then, hence there was no obvious solution (as there now is.)
Thus he was confronted, along with all Philosophers, with the Problem of the One and the Many;
Now there are several ways in which the one and the many are in opposition. One of these lies in the fact that the one and the many are opposed as indivisible and divisible. What is either divided or divisible is accounted for as a kind of plurality, whereas what is indivisible or not divided is said to be a unity. (Aristotle, Metaphysics)
Multiplicity cannot be derived from a necessarily single thing. (Aristotle, Metaphysics)
The solution is to realize that Space is Moving, that it is the Motion of Space which is the second thing that can Exist within the One thing. This Motion is a Wave Motion so we are simply saying that Space is Vibrating, Space is a Wave Medium and has Waves flowing through it. (So we see that Aristotle's required 'Activity' is in fact a Wave-Activity.)
Thus we see how the second existent, Motion, exists within the One Fundamental Existent, Space.
If we now return to Aristotle, we also see how the Wave-Motion of Space is consistent with his Metaphysical Principles, we need simply make two additions to his sentence as underlined;
And here we will have the science to study that which is just as that which is, both in its essence (Space) and in the properties (Wave Medium/Motion) which, just as a thing that is, it has. (Aristotle, Metaphysics)
Which thus explains why:
The entire preoccupation of the physicist is with things that contain within themselves a principle of movement and rest. (Aristotle, Metaphysics)
And thus we are now led to the Metaphysics of Space and Motion and the Wave Structure of Matter (WSM) in Space, and at last to a simple and elegant solution to the problem that the great Aristotle formalized so long ago. For Aristotle was largely correct, there is One Substance, Space, which Exists as a Wave-Medium (Aristotle's Prime Mover) and thus Space is Moving (Vibrating) due to this Wave-Motion.
Note: Space is exceedingly rigid and only allows a tiny displacement/motion, thus the velocity of waves is VERY fast (the velocity of light c) and the wavelength is very short (10^-12 meters) thus Space is vibrating at something like a hundred billion billion cycles/s.
As Einstein says;
The subtlety of the concept of space was enhanced by the discovery that there exist no completely rigid bodies. All bodies are elastically deformable. (Einstein)