It strikes me as contradictory that the scientific community will say that we don't know what dark matter is, but be happy to state things like "dark matter makes up about 85% of the cosmos" (source: phys.org)
Is there something wrong with the way I'm thinking about this? If MOND is correct for example, which seems to be a possibility still, wouldn't this mean that the statement about dark matter making up a certain percentage of the universe be false?
Ultimately, the presence of invisible mass is the most straightforward explanation for what we're seeing. We're aware that we could be wrong, but if we are, we're wrong about some of the fundamentals of physics.
We can see that there is an effect, and the only thing we know of that could cause that effect is mass, which is a property of matter. Something is changing the orbits of visible objects and bending light from behind it, among other things. Based on the influences we can observe, we can estimate how much mass is there and how it's distributed, and when we do that, the numbers suggest that there's a lot more mass that is "dark" -- not glowing and not blocking light to any significant degree -- than mass that we can see as "normal" stuff like stars and dust clouds.
I suppose it's possible there isn't any invisible, intangible matter out there. Maybe we'll figure out that there's some trick to how gravity works on huge scales that explains the discrepancies without requiring more than 3/4ths of the matter in the universe to be "dark". Maybe there's more than one explanation and what we call "dark matter" today is actually half a dozen distinct effects that we've incorrectly collected into one category. We'll somehow have to explain things like galactic collisions that appear to have separated the light and dark matter into two distinct blobs, both bending light but only one visible to our instruments, but -- sure. It's possible there's some way to handle it that doesn't require dark matter.
However, until we have a good reason to think that we aren't observing mundane gravitational effects from matter we can't detect, we're going to talk about it that way.
That's kinda how science works -- we often talk about the best current explanation as if it's true, while we all understand that it could potentially be overturned tomorrow. And why not? That's the state of all knowledge. Anything we know is subject to being replaced if somebody comes up with a new explanation that fits all the observations better than the old explanation and stands up to extensive scrutiny.
Let me use an analogy as an example of being able to say something about a thing we never see.
I can fairly confidently say that Zinn exist. Why can I say that? I've never met this person (or organisation, or AI) called Zinn! I can say that Zinn exist because he/she/it asked this question that I'm answering to.
I have no idea what exactly is Zinn. I call him/her/it Zinn because that is the self-described name in the user profile. But if this question was asked by an anonymous user I have no name for such an agent. Thus when discussing this question with other Stackexchange users I may need to constantly refer to the user as The anonymous user that asked about why it is OK to say dark matter make up X amount of matter which would be kind of annoying to say that every time. So I may just give such an anonymous user the "nickname" of "dark user".
I can make many confident statements about dark user. Such as their question has two paragraphs. Or their question contains a link. You may ask how can I make such assertion when I've never seen nor met this dark user? Well, because I can see the question on Stackexchange.
Similarly we've measured that there is something affecting the gravitational behavior of everything else in the Universe. We can't see what it is but it is making everything else (galaxies) behave in a way contrary to our math. And we can calculate the weight of this unseen thing thus the mass of it.
We can, if we want, call it "the thing that we cannot see but has a gravitational effect on everything else" but saying "the thing that we cannot see but has a gravitational effect on everything else" every time we want to discuss it is annoying. So we gave it a name: dark matter.
Just because we cannot see it does not mean we can't see the effect it has on the universe.
"Is there something wrong with the way I'm thinking about this?"
Fundamentally, yes, but it's a matter of the philosophy of science as much as the science itself.
The question makes the implicit assumption (also very common in science media) that things in science fall neatly into one of two categories: absolute categorical fact that is an eternal and unchanging truth, or open speculation. In reality, everything falls somewhere between these two extremes. Even something as long standing and universally accepted as Newtonian mechanics is, technically, not 100% true because we know that quantum mechanics and relativity "overrule" it in important ways. And yet, we keep using it to build cars and teach it in schools, not because it's absolute truth (a concept that doesn't exist in the epistemology of the scientific method), but because it makes empirical predictions that have been repeatedly verified to a high degree of accuracy while providing a conceptual framework for understanding multiple phenomena.
The current theory of how much Dark Matter there is and how it's distributed has not been confirmed to anywhere near the level of classical mechanics, but it has still far exceeded the level of speculation. That's why it's by far the dominant consensus amongst scholars, with alternatives like MOND being decidedly minority opinions. It explains multiple different phenomena coherently and makes predictions that have then been confirmed by observations. So, sure, I guess that discussions about Dark Matter should come with the caveat that this is only under our current best understanding of physics. While that might be good to do in science communication generally, the shorthand of omitting caveats is often useful when trying to communicate complex ideas to an audience that has neither the required expertise to grasp every nuance, nor much interest in the matter to begin with. But this is true for everything in science, calling Dark Matter out specifically for it is odd.
As for what Dark Matter is, the fact that there is no scholarly consensus on what it is a brilliant demonstration of the scientific method on the scale of a whole research community in action! Statements about the amount and distribution of Dark Matter can be empirically backed up, so people agree about the theories that makes the statements which match observations. But the composition of Dark Matter is not so easily corroborated by looking at the rotation curves of galaxies and the like. Some theories based on supersymmetric particles were favoured for a while because of their theoretical elegance and how they fitted in with existing particle physics, but have now been progressively ruled out in the last decade by null results from CERN. This is exactly how science should work, and how it makes progress.
That scientists say they are very confident in the accuracy of their theories about how much Dark Matter there is, but far less confident about what Dark Matter is made from, is a clear cut example of scientific honesty and knowing the limits of current knowledge. It's just a shame that "very confident in the accuracy of their theories" gets translated for popular audiences as "has entered the canon of scientific dogma" when such a thing does not (and cannot) exist in science.
This is a slightly more technical answer that should nonetheless illustrate the concepts.
Wind back the clock to 1845. You are an astronomer of that period, and you are familiar with Newton's Laws, which have been extensively verified to work in the lab. You observe Uranus and notice that it moves largely as Newton's Laws predict it to, but there is some disparity with the theoretical prediction. What could be causing the discrepancy? The two obvious explanations are:*
With option 2, you can go further: if you assume that Newton's Laws are right, then you can calculate more properties of the extra something: its mass, its orbit, and so on. But note Newton's Laws only say the how much mass the new body has. It doesn't say what the new body is. It could be a new planet, or it could be something else. This is how Neptune was discovered - it was a theoretical prediction based on Newton's Laws.
The situation with dark matter is similar. If you assume General Relativity is correct, then there's a discrepancy between theory and observations that can be resolved by postulating the existence of dark matter. You don't know what dark matter is, but you can calculate how much dark matter there should be (which is where the 85% came from).
Or you could say General Relativity is incorrect, which might also invalidate the need for dark matter. This also happened historically - Mercury shows precession that Newton's laws could not explain, leading some to conjecture that there must be another planet closer to the Sun than Mercury. The missing planet (Vulcan) was never found, and General Relativity removed the need for the planet. In the same way, if MOND turns out to be correct, then dark matter would be unnecessary, and the statement that dark matter makes up 85% of the matter content would be incorrect.
At present, most astrophysicists believe dark matter is strongly favored over alternative explanations, which is why they are happy to make the statements you allude to. The preponderance of evidence is in favor of dark matter** - but they might all get egg on their faces if alternative gravity turns out to be right.
*Of course, in practice, it's significantly more complicated. Confounding factors could be: perhaps Uranus's mass was estimated incorrectly, or the distance to Uranus was estimated incorrectly, or your observations of Uranus are incorrect because of some atmospheric fluctuation in the Earth, etc.
**I suspect what really convinces most astrophysicists is the sheer number of independent lines of evidence that can all be explained by standard cold dark matter. An alternative hypothesis might be able to explain any individual line of evidence, but explaining all of them with one framework is extremely difficult. No alternative theory has come close, but as long as dark matter is not identified, some people will continue to think about them.
We've extensively studied dark matter properties using the effects of its gravity. It's no different than the Sun, really. We've only know what the Sun is made of and where it gets its energy from for about a century, but we've known of its existence from the effects of that mass and energy for as long as there have been people.
The presence of a third family of massive quarks was theoretically predicted in 1973, based on clues provided by other particle physics experiments.
The bottom quark was discovered in 1975, but the top quark was not revealed until 1995.
I think this is a reasonably apt analogy. There are plenty of observational clues that something called dark matter exists. These include: the dynamics of stars and gas in galaxies and the dynamics of galaxies in clusters; the gravitational lensing of light by clusters; the temperature fluctuations in the cosmic microwave background; the primordial abundance of helium and deuterium; and the way that cosmic structure has developed as the universe has evolved.
Attempts to work out what this dark matter is have proceeded in a scientific fashion - ruling out possibilities one-by-one whilst also making attempts at direct detection. No proper scientist ever makes the claim that dark matter must exist, and there are plenty of scientists out there suggesting and pursuing alternatives. Most, if not all, of these alternatives offer a less comprehensive explanation of observations than the unpalatable hypothesis that we do not yet understand what 80+% of the gravitational matter in the universe is.
Your question is a "wag the dog" one. Observations indicate that there is mass which is otherwise unaccounted for (and more specifically, 85% of the mass present in the universe). This is, then, by definition, what "dark matter" refers to.
Several of the other answers here already make this point, more or less, but I feel compelled to post an answer which points this out succinctly.
Your question is actually spot on, and I disagree with the other answers here. There is indeed a contradiction. But the problem is not with the statement about the percentage of dark matter to matter, the problem is with the statement "We don't know what dark matter is".
We do. We do know what it is. It is that which makes up 85% of the matter in the universe.
The statement "We don't know what X is" is used FAR too much in science. It is a philosophical statement which can never be fully answered to exhaustion. We don't know what electrons are, not really. Are they fundamental point particles, emergent properties of a field, just a representation of an underlying mathematical symmetry, all of the above? We don't know what viruses are, are they nonliving, are they another branch of life, a different form of life entirely? We don't know even know what HUMANS are. Are neanderthals considered human, is a fetus a human, are genetically modified humans human? You can see how the question is essentially philosophical.
There is a lot we know about dark matter, there is a lot we don't. To say we don't know what it is simply because we are unsure of the exact subatomic makeup or relation to gravity, when we very well knew what things were for hundreds of years before we knew what gravity or an atom even was, is just silly.
There is indeed a contradiction.
Well, first off, given how contradictory recent studies have shown to be, MOND isn't exactly a theory that's backed up by much anymore. It is in kind of a foggy place at the moment. It could still be a possibility, as you have stated ; but there would be a lot of issues remaining before being able to consider it to its full extent.
Now as for just how one may be able to derive such a proportion despite their absolute ignorance regarding what this supposed (supposed !) dark matter exactly might be, or exactly how it should behave in the grand scheme of things ; you must remember that this is purely a theory, and at that, one that was derived from observation --as to fit it.
An offset between reality as we observe it to be and that which we compute through models based on relativity is present at most scales, and increases the greater the scale. In order to "fix" these discrepancies, some of the leading theories on the matter (pun intended) derived the existence of some kind of "other" matter --that had mass, for certain, as that is how it would come to solve the issue in the first place, but which had to interact differently with "usual" (or regular) mass ; hence our inability to quite simply observe it.
Knowing this, it becomes obvious where this proportion arose from ; as this supposed "dark matter" is used as a kind of fix for models involving relativity to coincide with reality, a certain amount of it (which varies depending on the exact theory you consider to be the closest match) and only that amount would "solve" the issue.
Looking at the way elements of a galaxy spin around its center black hole for instance, it becomes apparent that either the model is missing something (hidden variables at a macroscopic scale), or that there is more to what we are currently able to observe. The latter option is what dark matter variations aim to solve, through a type of matter that only loosely interacts with other kinds of matter.
Perhaps this could even be attributed to vacuum energy and relatively empty space's proximity to massive celestial bodies, stretching apart this energy of emptiness to different extents. (take this with a grain of salt, that last bit isn't backed up by anything as it is but a thought that has occurred to me which I haven't got to look into just yet)
