What is empty space made out of?

Some kind of "Substrate--Perhaps---  SPACETIME itself.  Energy and matter may be 4D WAVES in this substrate.  A Particle, for instance, might be a "Standing wave" or "Dimple" in spacetime, while energy might be a MOVING wave.-----Various particles and energies may be the result of different combinations of wave attributes, such as frequency, polarization, amplitude, etc.

Okay, full disclosure, I'm not a physicist, just a fan.  Let me start out by saying that space may be a human mental construct like darkness, cold, dryness.  Something that is used to describe what seems to be going on, but I'll take a stab at this.  Space seems to act a little like some kind of elastic fabric.  People used to think in terms of the Aether; They considered that there was this frictionless fluid, maybe a liquid, that permeated the universe, and the planets could whiz around in it without slowing down and falling out of their orbits, and that transmission of light (and even radio waves) was actually akin to some kind of special sound wave in the medium of aether.  To get a sound wave that traveled as fast as they had been measuring the speed of light, aether had to be super-duper hard, waaay above diamond (think neutron star) yet our atoms could slip around in it without resistance.  One guy set up this contraption with mirrors reasoning that if light propagated in aether like sound moves in water, then it would be slowed down (waves shortened) in whatever direction you were traveling through, and since we happened to already be whizzing around the sun at about sixty-five thousand miles per hour, he could just use that.  The light waves going side to side weren't any different from the ones going front to back, it didn't matter which way you were going, or how fast, everything just looked like you were sitting still.  Light wasn't moving through a fluid. 

  We don't really say what space IS, so much as what it DOES.

We can picture gravity as heavy things (in fact all things with any mass) "denting" the fabric of space like bowling balls rolling around on a trampoline.  Bowling balls, golf balls, wrecking balls etc.  If they roll by each other, they'll curve each other's path.  They'll curve a photon's path.  The speed of light, that's a tricky one, why is it what it is?  Man! Don't know.

  Now different properties, forces, (some don't like the term fields but hey...) may in fact be different ways of bending space.  Back to the trampoline with the balls rolling around on it, that's just a mental image of more than three or four dimensions flattened down into a flat 2-D sheet.  Electromagnetism, Strong, weak may all be bending (twisting? stretching?) space on different axes than gravity, and that's why they do different things.  You know, they have really detected gravity waves in a big way.  They were even able to convert them into audio. 

  There is a limit as to how fast anything, any intelligence, can move THROUGH space, think rolling along the surface, but there doesn't seem to be a limit as to how fast you can STRETCH space!

  Okay, the speed of light is this fixed value, and it doesn't change when you're moving around, Time Does.  Why does it always act like you're standing still?  Good question.  It keeps seeming like you could somehow trick relativity into divulging your true speed and direction through the fabric of space, but I don't think you can.  I don't think you can send six rockets off in the six different directions and see which one of them is the most time dilated, and which one is the least to see which direction you're already going. 

  If you drill a hole in a plywood table and pass a thick copper wire straight through the hole, and sprinkle the tabletop with iron filings, then hit the wire with a jolt of current the iron filings will jump into rings around the wire.  Hit that same wire with 27 million jolts per second and you have a C.B. antenna!  Now, it's basically the same magnetic field, it just doesn't get to travel very far before the direction of the magnetic field behind it gets lined up in the opposite direction, then back again, over and over.  It looks like there are these waves in space that are about 11 meters long, and you can see this if you bounce them off a big metal wall, there are points where they add up and points where they cancel out, but that's not the whole story.  Three wavelengths away, that's just a record on the fabric of space of what the wire was doing 3/27,000,000ths of a second ago.  Now what I don't get is how that is like the kind of photon you can see with your eye.  I've heard that it is only a matter of scale, but answer me this, why is it that the kind of photon that comes off of a dipole antenna spills out in a torus all over the place except straight out the ends(that part I understand) but the kind of photon that comes off of atoms as light shoots out in only one direction like a little E.M. bullet instead of a wavefront?  No, I mean that with, a photon, if you're slightly off to one side, you'll miss it, but a radio wave coming off of an antenna, if you move off to the side you'll still get the same wavefront (unless it's a microwave dish, but what's acting like a parabolic dish inside atoms?)

  I'm beginning to think that space really is only our perception of space.  We are made of atoms.  Atoms are a specific width and stack up to make structures of a general size, and at certain temperatures chemical reactions take place at fairly uniform rates.  Those structures are things like trees, bacteria, sharks, mountains, corn, houses, and people.  Those chemical reactions include our heartbeat, circadian rhythms, and our thoughts.  We tend to despair if something we want to try is going to take longer than our lifetimes, and the vastness of space presents that prospect.  The atoms in our bodies stack up to make our arms and legs, the basic measurement we always have known.  The spaceness of space is really only determined by the size of things that aren't empty space, like planets, or the interval it takes light to zip through it, but that interval is determined by electromagnetic forces whose rates are determined by yet more properties of space, So we are left measuring and defining a thing with itself!

 “Empty” “Space” is primarily made of the Dark Matter zxjq

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There was a pure energy that has no form, but can perform work , then being ignited to become big bang, from that on its expand to become universe. While on the process of becoming universe , the energy turn to be masses and then filled them in space and then these three entities ie; energy , masses and space then being melted in to time dimension, in such a way that each one can can not remain without the others . So the first question that mankind has to tackle is, where is the first amount of pure energy as mention above came from, if one can answer with reason , logic, rational and intellect , then mankind can go on to answer  , what is empty space made out of, or is the space really empty, while it was being made out of energy, masses and time dimensions, as explain above.

It's bosons literally shoulder-to-shoulder there.

Empty space is not a substance, so it isn't made of anything.  It's a combination of direction and distance.

If you consider plasma in space to be nonsense, then I'm afraid you will refuse to read answers to your question. You see, space is made of many things. It's the rarity, or better, very low density, that we think space as being "empty". To give you an idea, 1 gram of protons on Earth will occupy 22.4 liters of volume. And this gram contains 602,000,000,000,000,000,000,000, or 6.02^23, protons.

And now hear this, in a "cubic meter" of space, there could be only a handful of protons, and dust particles too. And by the way, plasma means particles which have lost their electrons; and protons in space lost their electrons. Plasma in space contributes about 50% of the mass of a galaxy (the stars which are made of plasma the rest). It's no nonsense at all. Plasma is considered to be the fourth state of matter and it abounds much more than the first three which are solid, liquid, and gas.

We have two main theories of physics. General Relativity and quantum mechanics/field theory.

General relativity, driven down to its most abstract level, implies that space is relational - it only has meaning in the relationship between the metric field (gravity) and the things that produce the metric field (matter/energy). This further implies that a Minkowski spacetime cannot exist since there is nothing to produce a metric field (ignoring things like no matter/energy but there being a non-zero cosmological constant).

At a little less abstract level in general relativity, space and time are exactly what the solution - the metric tensor - says. It tells you how to compute angles and magnitudes, geodesics, and two different definitions of curvature. Makes no other statement about what space and time are.

Quantum field theory is a background dependent theory meaning that fields exist in a kind of independent background spacetime that is ‘just there’. The background spacetime always has the ground states of the fields. The uncertainty principle forbids the fields from remaining at zero (classical) energy. So, where there is space there is still stuff. This really isn’t a definition of space but implies a strong connection since dark energy increases as space increases.

Then there is loop quantum gravity and all the other quantum gravities with their various characterizations of discrete spacetime structure.

No one knows.  Terms like "dark matter" and "dark energy" are batted around, some little bits of evidence, lots of theories.  Still, no one is sure.

One fact we know.  Space is a lot of nothing.  Or not.  But it is out there.

That's something that is always being researched.  A property of space is that any parcel of space can be measured in 3 dimensions, but also in what seems like totally empty space it isn't actually empty.  Vacuum energy causes the spontaneous generation of pairs of particles that materialize out of the emptiness, and exist briefly before meeting each other again and completely annihilating each other.  But as far as a question of what is the substance that space is made out of, that is either a nonsense question because there is no substance that it is made of, or if it is something then it's likely not been discovered yet.  You need to ask a quantum physicist.