The quantum cat.

so in short the "third state" is: We don't know, and we can't look without interfering with it.

the best comparison I can come up with is that it is very vaguely like checking your tire pressure - a standard tire pressure gauge changes the tire pressure (though only by a very tiny amount) when you use it, because it releases a tiny volume of air from the tire.
on the quantum scale, the same thing would happen only a LOT of air could be released.

Quantum physics and I have an understanding - I stay out of its business and it stays out of my business.

perhaps the best way of stating it is "I know I don't snore because I stayed awake one night and listened."

Dark matter, if we're talking the scientific use of the term, does not cover any and all types of matter that we can't directly detect, like black holes, planets orbiting other stars or rogue planets. It's a "material" in its own right.

Black holes (or at least supermassive black holes) emit energy. Quite a lot of it actually. Mostly in the form of x-rays and quasars. No, the black holes themselves are not directly visible, but the matter and energy that they both consume and emit is. Those are some of the things astronomers use to detect them. Dark matter doesn't appear to emit or consume any type of energy at all. At least not any that we're yet capable of detecting.

The notion that we can't directly see planets orbiting other stars is wrong. We can and we have. Yes, the first time we detected a planet orbiting another star was by gravitational wobble of the star itself, but we've since then come to a point where we can detect them directly by toning down the glare of the star by use of filters and physically seeing the planet create a shadow when it passes between the star and us. There are also systems to detect alien planets that rely on audio detection (the star itself emits auditory frequencies that are distorted when a planet passes between it and the auditory equipment - by measuring the distortion, we're not only able to hear that a planet is there, but actually able to know how large the planet is compared to its star). Dark energy doesn't create gravitational wobble, doesn't leave any shadows and can't be detected with auditory equipment.

Rogue planets (planets that have been ejected from their star systems, in case someone didn't know) are more difficult, but again, not impossible to detect. Floating around out there in interstellar space with no nearby stars that can light them up, there's little to no reflection to detect, but it's been hypothezised that an x-ray or hyper sensitive infrared telescope might be able to detect them. Again, dark matter doesn't interact with any type of light and IR and x-rays are just light waves outside the visible spectrum, so again, not something we can use to detect dark matter.

While all of these types of matter might not be immediately visible to us, we'd still at least be able to physically touch them if we were to encounter them while flying through space. You might be able to make out the event horizon of a black hole if you got close enough and if it's actively drawing in matter, you'd at least be able to see the accretion disc (matter "circling the drain"). If you flew into an alien solar system, the planets that orbit that star would be as visible to you as the ones in our own solar system. If you were passing through interstellar space and came across a rogue planet, you would be able to shine a light on it, detect it with radiowaves or see it with IR or x-ray filtered goggles/telescopes. If none of those are available or they don't work, you wouldn't doubt the rogue planet's existance for a second once you got caught in its gavitational field and crashed on its surface. None of these options are available to detect what scientists call dark matter.

The whole reason for coming up with the term dark matter in the first place is that something is pulling all the stars and planets and asteroids and whatnot together to form galaxies and it's not the detectable matter itself. There's no way the visible matter (or even known, invisible matter like black holes) a galaxy consists of can create enough gravity to pull it all together to form the discs and ellipses we see in the universe today and it certainly wouldn't be capable of making the matter at the far reaches of said discs and elipses spin around the galaxy at the same speed as the matter at the center, so there must be something else doing it. Something is keeping all that matter from flying apart and we don't know what that something is.

Since the only thing in the universe we know to have gravitational pull is matter, but there's no matter to be seen to account for the gravity that's clearly there, the term "dark matter" was coined to convey that we THINK it's matter, but we can't see it, so we can't be sure. It could just as well have been named "ronkeydonk" or "habberplast" or just "we don't know matter". It's a term that's been applied because we just don't know what it is. For all we know, it might not even be matter at all.

As far as I can gather from what you're saying, you still think the term "dark matter" applies to anything and everything in the universe that is not directly observable, but can only be detected via its interactions with other things. If that's the case, you've misunderstood something.

The scientific application of the term "dark matter" pertains to one thing and one thing only, namely the as yet unknown "material" that is causing the immense amount of gravity we know is there, but which can't be explained by any known type of matter, including black holes.

Astronomers and cosmologists have detected supermassive black holes at the center of most galaxies, but the mass of those black holes combined with the mass of the stars, planets and debris orbiting them doesn't produce enough gravity to hold together a galaxy. Astronomer Fritz Zwicky concluded as much back in the 1930's and coined the term "dark matter" to explain this extra gravity holding galaxies, local groups of galaxies and galactic superclusters together, but he was met with skepticism by his peers.

The theory wasn't brough to life again until the 1970's when two other astronomers, Vera Rubin and Kent Ford, set out to map the dynamics of galaxies, using a higly sensitive spectrometer that Kent had developed. What they expected to find was that the outermost stars of a galaxy move slower than the innermost stars, since the mass at the center of the galaxy should have less gravitational impact as you move further away from it, just as it is with the planets in our solar system. The closer a planet is to its parent star, the faster the orbit and the further away, the slower the orbit.

What they actually found was that the outermost stars move at the same speed as the innermost stars. If the galaxies were held together only by their visible mass, that would be impossible, since the velocity of the outermost stars would fling them away from the rest of the galaxy. The only conclusion that could be made was that something unseen was creating the gravity that allowed the outermost stars to orbit at those immense speeds without flying out into space. The idea of dark matter got new life and no matter how many astronomers tried to prove Rubin and Kent wrong, they just couldn't. Zwicky had been right all along.

Roughly 85% of the mass in the observable universe is invisible and indetectable by any means we have now. There are many theories as to what that mass is, but no one has the answer yet. All we know for certain is that there is something creating a gravitational force of some sort, keeping stars, planets, dust and gas together in galaxies, galaxies together in local groups and local groups together in superclusters. That something is what has been dubbed "dark matter".

Sources:

Wiki: Dark Matter

Vera Rubin and Dark Matter

Further reading: Ask Ethan: Are Black Holes Made of Dark Matter? (By cosmologist Ethan Siegel)

And we think we're so smart.

As SD likes to say: We're really only at a point where we're beginning to realize the scope of what we don't know.

according to oziris; no. my understanding of what he says is that "dark matter" is only the stuff that cannot be accounted for by planets, comets, asteroids, and such whether we have managed to spot them with a telescope or not.

my understanding of dark matter theory is that they added up the gravitational attraction of the galaxy, and the velocity of the stars orbiting the galactic core, and determined that the stars should be at escape velocity, yet they are not flying off into space. therefore there must be something generating extra gravity, which has not yet been discovered.

While exoplanets and black holes are "matter that is dark to us", they're not the same as the phenomenon that's been given the name "dark matter".

TLW and GTCGreg have it exactly right.

"Dark matter" is what has been determined as a gravitational force that keeps the stars in their galaxies, keeps galaxies grouped together in local groups and keeps local groups together in superclusters. Something is doing that and it's not any type of matter we've even theoretically been able to determine what is. We know there are exoplanets and we know there are black holes. We've had theories that said they were there and now we have evidence to support those theories. Not so for dark matter. All we know is that there's an unexplained (maybe) gravitational force holding everything together and we've named that "dark matter" for lack of a better term.

We don't know that it actually is ANY type of matter or that it's even gravity holding them together in the first place. It could just as well be some other "universal glue" that we have yet to discover, which has nothing at all to do with matter or gravity.

As GTCGreg says, what seems to throw most people off on this is the name itself. The fact that we've given it a name that involves the word "matter". I think his suggestion of calling it "residue gravity" instead makes more sense, but as I said, we might find out later on that it actually has nothing to do with gravity at all, so even that term might turn out to be misleading if we ever figure out what it really is.

Same thing goes for "dark energy", the force that drives the accelerating expansion of the universe. The term "dark energy" is just a name that's been given to the phenomenon, but we have no clue what it actually is. One working theory is that, just like dark matter, dark energy may be the gravitational pull of a single or even multiple parallel or neighboring universes, pulling on the matter in our universe. Another theory is that spacetime in and of itself possesses the energy to drive expansion. We don't know.

The point is, "dark matter" and "dark energy" are just terms that were coined for the sake of being able to name the phenomena in conversation while we try to figure out what they really are.