Nothing is a pivotal and indispensable concept for physic
Nothing in addition to the commonly used colloquialisms means lack of any physical object. Now it could raise another question: What does lack of any physical object mean? This can be answered by two words - "lack of space". If there is no space, automatically, no physical object 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 objects. So this means that space occupied by physical objects must be interspersed by places forming perfect emptiness. Otherwise, the universe would be one organism, an object merged into oneness. And here we conclude that there must be in nature the smallest physical object, from which derives every form of matter. At this point, one might ask whether the primordial building blocks of matter is not the matter itself? Definitely not. On the other hand, I would not mind calling this objects "pre-matter" because despite 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 ether before the creation of matter, now - resultant (dependent on the 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 "ether", 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, the fundamental unit of ether (unrecognized by "official" science but still sought so-called Dark Matter), would have a globular shape, with no inner structure and identical to the magnetic field. 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 explained in which "places" occur perfectly empty spaces, comprising NOTHING. And they are insulator, the smallest physical reality, allowing the existence of various forms of matter. 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 object and there is a 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 (or Nothing) gives the feasibility of the existence of Something, plays the most important role in the possibility of the existence of the universe. 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 ether, and thus physical objects interspersed with NOTHING or places of a pure vacuum. On the other hand, this part of the universe, which constitutes a limitation as to the possibility of its further penetration contains, in my opinion, only cosmic medium or ether, where there is not even a 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 ether objects
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 an onion). The universe is a real object of all objects. Real objects are imaginary and that means that you can, regardless of the scale and accuracy, sketch them on a scrap of paper. If the universe, which is a real thing 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 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 attenuated, which is for many reasons unacceptable (but that's a separate topic).
In summary:
NOTHING within the universe should be understood as areas of a pure vacuum. Such areas occur between units of ether, thus the existence of space is possible due to ether and empty spaces between them.
NOTHING "beyond" the universe means lack of space, thus there are no objects (including ether) "outside" the universe.
Philosophical postscript
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 the magnitude of ether (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.