The term “dark matter” has a bit of a connotation that this is something special - a different kind of matter than we have observed before. Perhaps a better term would be “non-luminous matter” – ordinary matter that isn’t, like the stars, heated to incandescence.
I’m not saying that because of my suspicion that dark matter is the new luminiferous aether, that physicists are stupid or are doing something pointless. Just giving my hunch as a non-physicist who is interested in science, that’s all.
That said, I shall continue in this vein, with a couple questions for the physicists:
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What if we can never really figure it out, because the universe is too big and are spaceships will always be too slow to measure things outside of our tiny little neighborhood? You can add in the inherent limitations of our ape brains; although artificially intelligent computers might someday transcend that limitation and be able to learn things we cannot learn on our own or perhaps even comprehend.
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From the other angle, maybe our entire observed universe is really just a tiny fraction of the true universe. Maybe everything we see is influenced by other actions that are far beyond our ability to see or measure. Maybe we live in a “microverse” and the Big Bang could have been a spark created by the spark plugs of an incomprehensibly large lawnmower.
ETA: Cadfael, I had always assumed until yesterday that dark matter was in fact just a bunch of ordinary non-luminous matter like asteroids or whatnot. But as noted upthread, apparently there are reasons to believe this is not possible. So actually it is more special then I ever suspected, if it even exists.
I personally prefer “atramentous corporeity” and “crepuscular palpitation” for dark matter and dark energy respectively, offering, as they do, a more baroque and slightly late Victorian flavor to a field that has becoming increasingly dry and tending toward acronyms in its jargon.
[THREAD=566909]This thread[/THREAD] has an extensive discussion of MOND theories versus the application of (missing) dark matter as a correction factor.
Stranger
Although we will likely not be able to travel to locations far outside the Solar System to make local observation for the foreseeable future, we can make observations of the large scale structures in universe across tens of billions of light years. While this gives us a constrained view somewhat akin to the allegory of Plato’s cave (i.e. having to infer things that we can only observe remotely and with limited fidelity) we have been able to discover a lot about fundamentam mechanics of the universe on both large and tiny scales despite not being able to directly observe or interact on those scales.
As far as “the inherent limitations of our ape brains”, we’ve already exceeded those limitations in the use of high performance computing and methods for munging and visualizing massive data sets, which simply could not be done by single individuals or even entire teams in any reasonable period of time. While this is not generally what people think of as “artificial intelligence” it certainly serves to amply our intelligence and provide basic insights that we would be incapable of achieving on our own. This is hardly new, of course; this is a trend which started with the discovery of the logarithm, which allowed for simplifying complex problems into more simple ones, thorugh trigonometry, series expansions, differential equations, integral transforms, statistical methods, and finally modern numerical methods for handling matrices and tensors, and will no doubt continue with increasingly sophisticated methods for processing large data sets and identifying representive trends. The intuition and insight provided by our “ape brains”, however, is still necessary to tie together disparate phenomena and methods, which are general tasks that even the most advanced computers are not very good at doing on their own.
As far as we can tell, the extent of the Universe beyond the apparent cosmic horizon may well be infinite. However, we have the expectation that the same physical laws apply everywhere. This may or may not actually be true, but it has allowed for a consistent model of gravitation (general relativity) that appears to be in agreement with observation on most scales save for the fact that we cannot observe the apparent mass and energy required to achieve balance. This answer is certanly cleaner than any attempt to modify GR to explain differences of behavior on different scales, which at least at this point would appear to be competely arbitrary.
Stranger
Yeah, my grandfather was a civil engineer for the federal government who oversaw several large projects like reservoirs and bridges. He told me that before computers, they would send problems to be solved by a roomful of mathematicians who broke them down into parts to solve them and then telegraphed back the results. Really shows what a true laboursaving device the computer is. (He was pained too that despite my mathematical aptitude, I did not know how to work a slide rule.)
BTW, I cringe at seeing “are” written in place of “our”; I use an iPad to compose most of my posts with the dictation feature, and I missed that erroneous transcription when I edited.
Surprisingly easy to learn, and fun. It gives you the same kind of joy as doing large multiplications out of a book of log tables, and the same kind of connection with history as knowing Roman Numerals.
Mastering the slide rule is a little like reading Moby Dick: you do it because it’s out of date. It connects you to your heritage.
(I just did some exploring, and learned that there is a slide rule app for Android smart phones. GOTTA HAVE!)
Gods I feel old. When I was going to college to get my degree in AS engineering, we used slide rules (frequently), and I have several in a box somewhere…along with my first HP calculator. :eek:
This is an odd topic for a “Great Debate”, since the question in the OP, “why are scientists so sure dark matter exists”, has a factual answer. Basically, scientists made a bunch of independent measurements of a bunch of different things (as detailed by Pasta in post #26), and all of these are consistent with there being a bunch of invisible stuff out there that exerts a gravitational pull. We don’t know for sure what it is yet, but the observations pretty clearly indicate that there’s something there.
I wonder if the OP is convinced yet? If not, what do you think the correct interpretation of all those experimental results is?
Let’s turn the question on its head: Why would one think that dark matter does not exist? Doesn’t it seem like rather an extraordinary assumption to think that all matter must necessarily couple to the electromagnetic force?
Hey, I pay my rent quite regularly but have never been tested to the highest precision! 
Right. We basically have a few options:
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General Relativity is correct, but we have to add this dark matter stuff.
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GR is mostly correct, but we have to add this other stuff that’s quite a bit more complicated than dark matter.
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GR must be wrong, back to the drawing board.
Until there’s an option 4 that explains everything, it’s the best we got. Meanwhile, GR has been amazingly accurate in its most striking predictions, and has enormous explanatory power. No surprise that we cling to it like a raft in a storm.
As mentioned earlier, the luminiferous ether wasn’t a useless concept, even though it turned out to be wrong. It was what was needed to make the old theory work, and it had testable ramifications. Dark matter is similar in those two regards. Only time will tell whether, like the LE, it’s wrong.