Nothing in addition to the commonly used colloquialisms, means lack of any physical entity. Now it could be risen another question: What does lack of any physical entity mean? This can be answered by two words - "lack of space". If there is no space, automatically, no physical entity can exist. But there is also a second form of NOTHING. Well, it is a complement to the entire volume of the universe which contains all physical entities. So this means that space occupied by physical entities must be interspersed by places forming perfect emptiness. Otherwise the universe would be one organism, an object merged into oneness. And here we come to the conclusion that there must be in nature the smallest physical entity, from which derives every form of matter. At this point, one might ask whether the primordial building blocks of matter is not matter itself? Definitely not. On the other hand I would not mind to call this entity "prematter" because in spite of its invisibility it bears features of matter - can be detected, but the essential difference is that the feature of matter is rotational motion, absent in aether before creation of matter, now - resultant (dependent on matter). In addition to that it is also the medium (conductor) allowing interactions of all forms of matter. Nevertheless, I stick to authorized naming, i.e. the concept of "aether", which has its historical tradition, as Aristotle foresaw the necessity of its existence, and later, among others, Maxwell and even Einstein, who contradicting his theory in 1920 stated that "...space without Aether is unthinkable...". Without going into other speculative details which I discuss elsewhere, fundamental unit of aether (unrecognized by "official" science), would have a globular shape, with no inner structure. A feature of globes is, that when coming into contact with each other, create free spaces between them as opposed to, e.g. cubes. By this lengthy introduction I came to explain in which "places" occur perfectly empty spaces, comprising NOTHING. And they are both an insulator allowing the occurrence of various forms of matter, as well as being the smallest physical reality. The word "places" I took in quotes because (using commonly known notions) location "beyond" the universe cannot be called a "place" because there is no physical entity and there is lack of space "there". Thus NOTHING belongs to pivotal concepts in physics - it's so important, as important is ZERO for mathematics. Something that materially does not exist and cannot - as it could seem - give feasibility of the existence of something, plays the most important role for the possibility of the existence of the universe we inhabit. In other words, NOTHING has its due "place" in the Universe. Summing up, cosmic space comprises SOMETHING. Because it contains in addition to the wealth of forms of matter, filling it aether, and thus physical entities interspersed with NOTHING or places of a pure vacuum. On the other hand, this part of the universe, which constitutes limitation as to the possibility of its further penetration contains, in my opinion, only cosmic medium or aether, where there is not even presence of a cosmic microwave background radiation. So, where should we look for NOTHING then? The answer to this question is simple:
1. In the universe - spatial insulation between aether entities
2. "Beyond" the universe
And why is that? Because the universe is finite. If there wasn't NOTHING, the universe would have to be infinite (in the sense of endless layers of onion). The universe is a real entity. Real entities are imaginary and that means that you can, regardless of the scale and accuracy, to sketch them on a scrap of paper. If the universe, which is a real entity would be infinite, as some believe, then it couldn't be imagined and thus sketched. And what does it mean? Now that the infinite universe cannot exist, because every real thing we can in some way imagine; describe. I'd like to observe at the same time, that the notion of infinity has its useful place only in mathematics. In nature does not occur. And this is for the simple reason that infinity should be understood as a sequence, i.e. process of continuing of something that is ready (finished), without the possibility of seeing the final (finite) wholeness. In other words, it is a continual string of something concrete, following one by one, e.g. decreasing or increasing sequence of numbers. In the case of the universe, it would be its incessant conquest of NOTHING, contents of which would have to be more and more attenuate, which is for many reasons unacceptable (but that's a separate topic).
NOTHING within the universe should be understood as areas of pure vacuum. Such areas occur between units of aeter, thus existence of space is possible due to aeter and empty spaces between them.
NOTHING "beyond" the universe means lack of space, thus there are no objects (including ether) "outside" the universe.
Certainly it is extremely difficult to imagine such an "empty" space that could end sharp, because humans cannot observe the micro-world from the perspective of magnitude of aether (resemblance to a dense fog). Although it is difficult to imagine, it must be so as there is no possibility of reducing the space to the half-space or its fractions. And it is difficult, because the humans are not experiencing ideal NOTHING and the LACK of SPACE. Things that are unnecessary, and even more, impossible, we do not experience.