Is space actually a vacuum or it is so large that any matter is negligible?
'vacuum' is used fairly loosely. Essentially, its common use is merely 'where there is negligible matter'. We say 'it is a vacuum' in orbit, or on the Moon or between planets, but it isn't really. All these places are peppered with gas and dust, but the density is so small we tend to call it a vacuum for most intents and purposes (such as whether we can breather there, or if water will boil).
'space' is also an ambiguously-defined term. Space is not a thing that has a volume, unless you define the volume.
I think you're asking if the 'observable universe' is essentially a vacuum. Asking if the universe is a vacuum because it's so empty is like asking 'Can a box of tennis balls be considered empty if you ignore all the tennis balls in it?'
Although there is gas and dust and other particles present in minute amounts, for the most part interstellar space is a very hard vacuum.
The density in star forming regions is marginally higher, but still is very tenuous, here the density is enough to block some light.
These are the dust lanes and nebulae that we see in images of galaxies.
Regions where it gets dense enough to collapse into a new star are rare, but it does of course happen.
Its a little like asking how dark is dark. Generally speaking interstellar space has a harder vacuum than the best we can produce on earth.
Even an absolutely perfect vacuum isn't really empty as far as we know either.
Now, Dave, be fair. It's a lot more like asking 'can you consider a cargo container empty if there are a couple of grains of sand in it and you ignore them?'
Damn, I think you just came up with a solution to the paradox of the heap ... well, kinda, anyway.
You just... ignore them.... lol...
The OP's question has been addressed. Thread closed.
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