Personally, I’m agnostic as to whether a vacuum can or does exist. But if I were inclined to argue for the existence of a vacuum, I might do it like this:
The mean density of interstellar space is a few hydrogen atoms per cubic centimeter.
Okay, take a cubic centimeter of interstellar space with four hydrogen atoms in it.
So, there: Your vacuum is all the space in the cc that isn’t occupied by the four hydrogen atoms.
But things are never that easy. So what’s wrong with this argument?
I think the difficulty lies in determining exactly where in your cc of space those four molecules are, and where they’re heading.
If vacuums did not exist, why would we have things to clean them? 
I thought this was going to be: “If a vacuum is nothing, how can it exist? Because a nothing is like, nothing, right?” Shut up, Parmenides.
Seriously, though, isn’t the reality of vacuum kind of basic to modern (i.e., from Galileo onward) physics?
Nature adores a vacuum.
I had no idea there was any controversy over this. Are there people arguing against the existence of a vacuum? What are their arguments, and what is their definition of “vacuum”?
Does the same apply to intergalactic space, or is that even emptier?
+1
Interstellar and intergalactal space are not complete vacuums. Some particles do exist in this large “void.” But any specific cubic volume of this void may not at any particular time contain any matter. So, for a specific time for a specific volume, no actual particles may exist; nonetheless, virtual particles are being continually produced, coming into and out of existence.
Do virtual particles count, though, or even regular subatomic particles? I mean, photons are particles that are not even virtual. Is some volume of space not going to count as a vacuum just because it has light shining through it?
“Vacuum” is usually defined as space being completely devoid of matter. Of course, you are welcome to your own definition.
If I want to know if a vacuum exists, I usually look in the cupboard under the stairs- I usually don’t.
More seriously, how big does this vacuum have to be and what are the properties that define it? A perfect vacuum is generally speaking idealization, it seems unnecessary to worry if such a thing can genuinely be found in nature.
Well, it would appear that there is controversy over this. On the one hand, we have someone taking it as a given that vacuums exist, presumably in nature. On the other, we have someone talking about vacuums as though they were ideal objects, presumably not to be found in nature.
I guess I’d think a vacuum would be a space of indefinite size devoid of any type of material entity, where ‘material entity’ includes any of the objects of science (physics, specifically). No virtual particles, fields, photons, nothing.
“Vacuum”, in this context, is generally taken to mean the lowest-energy state of that sea of virtual particles: There’s a certain amount of virtual-particle activity which is tolerated within this definition. It is not completely certain, however, that what we have now is actually the true vacuum: It is within the realm of possibility that we currently only have a “false vacuum”, with the virtual particles being in an almost-stable excited state. If this is true, then it would be possible for some event (what, exactly, nobody knows for sure, but it could even just be a random fluctuation) to cause a small region of the vacuum to drop down to the true ground state (or to another metastable state in between), which effect would presumably propagate out at the speed of light, and end the Universe as we know it without warning.
This is related to physisists thinking that space is a thing in its self. It expands when matter is there to occupy it but there is nothing on the other side of that space. Sounds like to me they are just trying to claim understanding on something they don’t understand. I say space is space and it goes on for eternity. If there’s nothin there then it’s space!
I hate when that happens.
I don’t understand what you’re trying to say here, and it doesn’t particularly resemble any idea discussed by physicists. Where did you get this notion from?
Is this what I’ve seen mentions of “quantum foam”?
I think that is the definition I was using. What was at issue was what counts as matter. Do you think photons are matter? Do you think virtual particles are matter? If yes to one and no to another, where do you draw the distinction?
I think he is probably talking about the expansion of the universe, and of space itself, from the big bang, and the notion that space itself is expanding, rather than the universe’s matter expanding into pre-existent space… It is a bit garbled, but it does at least resemble an idea discussed and (unless I am very out of date) even generally accepted by physicists.