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So I have been thinking about this question for like five minutes. Is ‘space’ really empty? I know that technically there aren’t atoms in deep space and that would make it empty, but there has to be something to fill in those those gaps, right? So technically space isn’t empty?

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    $\begingroup$ There are some atoms, practically everywhere there are a few. But even atoms are mostly empty. And that empty space is empty. $\endgroup$
    – Topcode
    Oct 3, 2022 at 16:11
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    $\begingroup$ Yes, space is pure emptiness. That's why, in my and many other peoples view, the universe must be literally infinite since emptiness can't just stop. $\endgroup$ Oct 3, 2022 at 17:39
  • $\begingroup$ Then what fills in the emptiness? Dark matter? If so, what’s dark matter? $\endgroup$ Oct 3, 2022 at 19:40
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    $\begingroup$ @PrincePugs 'there has to be something to fill in those those gaps, right?' What makes you think this? $\endgroup$ Oct 5, 2022 at 19:26

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No. Space isn't empty. There are atoms. The density varies quite a lot, from a million atoms per cubic centimeter to fewer than one per 100000 cm³. Mostly this is in the form of hydrogen.

And there are photons, neutrinos, dark matter (whatever that is), electromagnetic field...

Moreover on a larger scale there are particles of dust, and of course at an even larger scale, there are stars and planets in space.

Space is much more empty than the atmosphere, but it isn't completely empty.

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    $\begingroup$ +1, but note that 10,000 cm⁻³ is dense in astrophysics! $\endgroup$
    – pela
    Oct 4, 2022 at 15:55
  • $\begingroup$ Ah, sorry, I read it as "10,000 per cm$^3$". You're right, one per 10,000 cm$^3$ is not dense (although still an order of magnitude denser than the average density). $\endgroup$
    – pela
    Oct 7, 2022 at 20:48
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While I believe this is a matter of semantics, in a meaningful, physical sense (with regards to quantum field theory) space is never really empty. Quantum fields exist everywhere, and particle pairs spontaneously come in and out of existence all the time, with effects that are measurable (e.g. vacuum polarization screening on particles). These fields fluctuate and correspond to the elementary particles that we are familiar with, such as neutrinos, photons, electrons, etc. they carry energy, and some have a vacuum expectation value that is non-zero.

Photons fill what would otherwise be considered ‘empty’ space as well, with at the very least the CMB being (almost) isotropically found.

In essence, while an area may not have macroscopic objects that you can see with the naked eye, I think one could comfortably say there has never been a ‘truly’ empty part of the universe, ever, because of the presence of these quantum fields.

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    $\begingroup$ @PrincePugs This is a good answer, but note that good old atoms are still ubiquitous in space; see James K's answer. $\endgroup$
    – pela
    Oct 4, 2022 at 15:57
  • $\begingroup$ @pela Yes, I know. I meant about the space that filled in where there wasn’t any atoms. $\endgroup$ Oct 26, 2022 at 20:56
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Well there is radiation from CMB, maybe near a galixy a magnetic field, you have virtual particles, here is an interesting video

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