The Universe is a continuous plenum of Existence, containing no voids. A void would constitute an area of nothingness, a non-existent within the realm of Existence. Such a concept is an inherent contradiction. Indeed logically, “a non-existent” is what I’ll term an improper noun – it has no referent. [I run the risk here that there is a prior definition of “improper noun”, but for this post I’ll take that risk].
The difficulty in integrating the statement that the Universe is a plenum often lies in a confusion between a void and a vacuum. A vacuum is an area of space containing no matter, while a void contains nothing with an identity – this is a far greater restriction than a lack of matter. Vacuums can and do exist in the Universe, while a void cannot.
Bringing this to a concrete example, consider two spheres not in contact with one another. By “not in contact” is meant there exists a distance between them. A distance is a difference in position. To have a difference in position there must be “space” between them. This space may be occupied, with matter, or not occupied, as is the case in a vacuum. In either case, the space itself is a non-void, and existent.
A description of the structure and behaviour of space is in the realm of Physics, and beyond the context of Philosophy. Nonetheless, a consideration of some of the attempts to represent the Plenum in physics can serve as illustrative examples that can help the process of integrating the concept of a vacuum as an existing entity, and therefore completely different in principle from the non-existent void.
Here I will mention only briefly four representations of the Plenum. In pre-Relativistic physics, we have the Ether described as a “fluid” through which electromagnetic waves propagate, permeating – consisting – of all of space. In “modern” “accepted” physics the concept of the Ether is replaced by the quantum field. This field is the substrate for not only electromagnetic propagation, but all energy and matter in the Universe. There are severe philosophical issues with the quantum field (it itself relies upon false concepts of the non-existent), but it is again identified with all of “space”. Lastly, I’ll list two alternatives to the accepted theories. The elementary wave theory of Lewis Little presents an approach to modern physics which rests upon firmer philosophical grounds, and has been accepted by many Objectivists as an alternative to quantum mechanics. Finally, there is the view of the Universe as computation. In this framework, the Universe consists of a vast array of computational cells, plus a “program” of evolutionary rules which determine the content of these cells through time. This alternative framework has been pursued by Edward Fredkin, Steven Wolfram, and others.