According to quantum field theory particles are just fluctuations in fields that permeate all of space, so sure.

(The “fabric of spacetime”, on the other hand, is more of a mental analogy than an actual thing.)

Nothing doesn’t exist, by definition.
The concept definitely does.

Time to rinse off and get out of the shower.

You can't have a space with dimensions if it doesn't exist in spacetime, so the commonplace understanding of nothingness is a space within spacetime that has nothing that we can interact with that's interesting to us. People will say that there is nothing in the desert, but there is lots of stuff there. People will say there is nothing in a room, but there is still air. People will say there is nothing in space but there are still diffuse atoms, gravity, radiation and virtual particles. If your definition of nothingness is that there is a space with dimensions and time that has absolutely nothing in, then yes that does not exist.

I'm 14 and this is deep /s

Historically, "nothing" is the label we apply when none of the things we are currently able to detect are present.

After we can detect what actually is there, we update our description to the much more accurate term "practically nothing".

It's a concept I've been thinking about for months or even years, the concept of non-existence. In my mind I can sorta visualize it, but I'm not able to transcribe it to words, I'm not able to start explaining it because whenever I try to start writing something, it starts morphing into existence. For example: a phrase I can think of is "Light needs a darkness to shine unto", it sounds like it can describe the concept, but then science comes out of nowhere to slap me in my face with the understanding of how matter emits radiation and how there's no such place as "completely absent from any radiation".

In my mind, the complementary makes sense, substance needs substrate which needs the substance, light needs darkness which needs the light, Hadit needs Nuith which needs Hadit (the infinitesimal point needs the infinite circumference which needs the infinitesimal point), and so on. See, human language is made to conceptualize what can be conceptualized, and non-existence is not conceptualizable in essence. However, the existence needs a counterpoint, a counterpart, something to contrast with its conceptualization, because if there was only existence, there'd be no existence at all (how can we conceptualize a thing if it's the only thing wherever you look, wherever you go?). We can conceptualize the fabric of spacetime because "it's there" and, by "there", I mean "there" as in "where the fabric of spacetime sits on", just like the shine of a spotlight illuminating a place where it was shadowy and dark.

There are things that we do know, there are things that we don't know yet but we can know, and there are things that can't be known. Who is the first Sumer person to ever write, what was his name, when he/she was born and when he/she died? What about the person who discovered the fire, who exactly were he/she? We don't know, we can't know, but they existed because now we have fire and writing systems. The impossibility of determining them doesn't rule them out of existence, just like the non-existence itself. I mean, it's the very essence of the non-existence to "don't exist" but that somehow makes it "existent", somehow the state of non-existence is a state, therefore, it exists as a state of being (as in "not being").

To make matters worse, the human language is made to describe things within the realm of existence, time and space, when and where, while transcendental concepts can't really be described through it without losing its transcendental essence. Non-existence is such a concept, a non-conceptualizable concept, so paradoxical in its nature.

If something isn't there, it must be there because we know it isn't there. By knowing it doesn't exist we bring it into existence by thinking about it. But when we forget, it actually ceases to exist.