Let’s say I was able to take an empty jam jar beyond Earths atmosphere, then took the lid off and after a few moments, replaced the lid. Would there be anything in my jam jar that could be analysed?
Space is mostly vacuum. You might a find a few atoms of residual gas, plus a few stray atoms or molecules of whatever happen to find their way into the jar, but little else. In interplanetary space on average you can expect to find a few hundred particles of matter per cubic centimeter of space; in intergalactic space, this number approaches a few particles per cubic meter.
Neutrinos, photons, and gravitons; maybe a few spare hydrogen or helium atoms. Of course, the non-fermion stuff is just going to flit right through (I’m assuming the jar is transparent), which leaves you with basically nothing. The stuff that is spacetime isn’t stuff, although all the above stuff is embedded in it. You can’t grab space, or inspect it, or measure it directly; we can only infer its emergent properties by how things react within it.
A lot people used to claim that space was full of an invisible and immiscible fluid called the luminiferous aether which provided a medium for light to travel through, but then some obscure Swiss patent clerk came up with a crazy theory that motion is all relative and the speed of light is invariant in all directions and by all observers. You can imagine how well that went over. He also claimed that light was a particle, despite the fact that it clearly acts like a wave. Some people from Sweden gave him a nice dinner and a little money in hopes that he would just go away, but that just spurred him on, and then we had all sorts of nasty business about locking cats in boxes and quoting from James Joyce, and it all went downhill from there. You can’t even talk about physics in polite company any more, thanks to these guys.
Stranger
So, is the stuff that these atoms are ‘floating’ in, the mysterious “dark matter” that has been mentioned in places?
It’s not matter at all. It’s empty space. Vacuum. There’s nothing there.
That depends on your definition of “nothing.” At the most fundamental level, the fabric of spacetime is believed to be a sort of quantum foam; a sea of virtual particles continuously appearing literally out of nowhere and disappearing just as quickly. This forms the basis for the so-called Zero-point Energy and the source of the Casimir Effect.
Well, if “nature abhors a vacuum”, is space somehow ‘unnatural’, or just beyond our current comprehension? Surely it must consist of ‘something’?
Thanks. Bad day. Needed that. 
It’s all a part of the service.
Stranger
“Nature abhors a vacuum” is just a saying, not a scientific observation. To make it scientifically accurate, one could say “Nature prefers a vacuum”, as the vast majority of the universe is simply empty space/vacuum.
As mentioned earlier, on a quantim level, even space time may copnsist of a quantum foam in which particles pop in and out of existance in infinestimal small periods of time, This will never be observed directly, as the brief "instant: that theyse particles “exist” for is so tiny that they have no chance to interact (ie be measurable) with the rest of the universe. The Casimer effect theorises that in certain situations, these particles can be induced to “hang around” long enough to interact, but in doing so they no longer are a part of the quantum foam.
Regards
FML
Yeah, but with a few exceptions, virtual particles are just that; a particle and its anti-particle are created and then destroyed before they have any influence on anything else, which leaves nothing.
As for (missing) dark matter, nobody really knows what it is; it’s basically a placeholder for a certain amount of mass that has to exist but that we don’t observe. There is a small amount of dark (i.e. non-radiating) matter, like you, me, and the cat, that we can observe directly, but it is not enough to explain the bulk motion of the galaxy, and adding more normal dark matter is problematic because we’d expect it to block more light (if it is dust) or condense into observable chunks (if larger), so most people think that there is something else that is inertial but only weakly interacts with light and electric charge. If we knew what it actually was, we could come up with a better name for it; personally, I like “atramentaceous corporeity,” but I haven’t been able to get it to catch on.
Stranger
“Nature abhors a vacuum” is one of those old-time sayings that we should try to get rid of. It’s a product of pre-technological societies and today it actively hinders understanding.
It was once difficult to make a vacuum because pumping techniques were primitive and sealing techniques even more so. For similar reasons, it was difficult to keep a vacuum, because pressure differentials were likely to damage any containers with an appreciably-sized one.
Branching off of these practical issues was the philosophical notion that a vacuum means absolute nothingness. Engineers understand that vacuums are continuums that start with few particles in a space and get progressively better until there are hardly any particles in a space. This is still very far from absolutely nothing. But philosophers liked to imagine “ideal” objects to do thought experiments on, and so fixed the idea that an ideal vacuum was literally nothing, not just a space with particles extracted.
The concept of a quantum foam of seething particles creating a vacuum energy destroyed the philosopical ideal. This “something” in a vacuum is the source of endless confusion from people, e.g. those who ask how the big bang could start from “nothing” and rely on the old concept of vacuum to insist that nothing can’t be anything.
It would really help to use a different word for that vacuum energy. But if we can’t change the meaning of “dark matter,” which we haven’t even found yet, we’re never going to change as common a word as vacuum.
We can ditch “nature abhors a vacuum,” though. And should.
The idea that “nature abhors a vacuum” comes from Aristotle, who died in 322 BCE. Cite.
Science has advanced somewhat in the 2,300+ years since. To be fair, most of this advancement has occurred only in the last 400 years or so, but nevertheless, using Aristotle as a starting point for scientific questions is more than a little behind the times, to say the least.
And why must space consist of “something,” anyway?
Let me pose another question for you, that I used to ask of my chemistry students years ago. In an atom, what is in the space between the nucleus of the atom and the electrons surrounding the nucleus (besides the electrons themselves)? Here’s a hint: the answer is not “air.” 
If you want to give a name to the unmeasurable ‘stuff’ of outer space, just call it Aether.
Cheers. I won’t be using any more of his quotes in a hurry!
When it comes to atoms though, is there only so much chemistry can analyse before it is handed over to the physics boys and girls? I was quite happy with atoms being described as tiny billiards balls. For all practical intents and purposes, is there any real need for quarks, mesons and all the other bits that go into building an atom?
Just to add one more nail in the coffin of the saying, I think it is accurate only in limited frames of reference, like the surface of a planet. Aristotle, of course, knew no other.
Thanks, I always thought it was Niels Abhor 
wasnt the “Nature abhors a vacuum” phrase directed at nature as in planet earth, animals/plants and things?
seriously how the hell would anyone from 300bc even know what a vacuum was?
I always took it to mean that nature will find away to fill any void, as in, if you were to remove a species another would eventually move in to take its place.
that was my take anyway, I dont recall ever hearing it refer to an actual vacuum
Well, they didn’t have direct experience with vacuum, (unless you count partial vacuums from things like straws - surely somebody had invented a straw by then, right? 
) but apparently that didn’t stop Greek philosophers. Look up ‘Platonic realism’ for a fascinating theoretical philosphy built around the idea that the only things that had true reality were ones that we, stuck in the physical world, could never even perceive. But anyway…
The idea of there being spaces in the world where there is nothing isn’t that big a conceptual jump, and apparently Aristotle debated with Atomists who argued that everything in the world was made up of tiny atoms of matter with vacuum in between - a surprisingly good guess for back then. So maybe it was one of those guys who coined the term, and Aristotle was basically just going “Nuh-uh!” 
Speaking of which, does anybody know just what the ancient greek translation of vacuum would be? Every etymology I’ve found for our word only goes back as far as the Latin.
I’m not sure if this is what you mean, but PET scanners used for medical imaging rely on the behavior of antimatter (positrons, in particular). Understanding the makeup of subatomic particles is definitely necessary to build this type of technology.