I wonder if "dark matter" could be placed on the list with Santa and dragons. Unlike Santa and dragons we tend to believe something like dark matter actually exists, but outside of a theory explaining rotations of galaxies that needs it to be there, there is no evidence for it that I am aware of.

Evidence (if any) is some sort of "positive" fact : "Evidence, whatever else it is, is the kind of thing which can make a difference to what one is justified in believing or (what is often, but not always, taken to be the same thing) what it is reasonable for one to believe." This morning I was sleeping at 9:00 and thus I had no evidence of the sun rising; this does not mean that I've evidence that the sun did not raised this morning.

I think in the absence of evidence to the contrary we should proceed with the working assumption that Santa and Dragons are no more than imaginary beings found persistently in mythological traditions. books.google.com/… Link is to Transactions on Rough Sets II: Rough Sets and Fuzzy Sets

Russell's version of Santa and dragons was the teapot orbiting the Sun. We are not supposed to conclude anything about such things, or even concern ourselves with them, the burden of proof is on those making the claims. They must present evidence of their presence. Absence of evidence (to the contrary), or even its impossibility, is not evidence of presence either.

To me "absence of evidence is not evidence of absence" sounds simple and clear. I don't really understand what potential problems can arise there. Maybe one could give more detailed examples of what could go wrong there?

The first problem you run into is defining non-existence. Unless you define something's non-existence you are not able to prove it. Usually when you define non-existence of something you are doing it by using the existence of something else. Just try it. It is exceedingly difficult to create a specific definition of sth non-existing. Sometimes people define nonexistence as absence and then you are on a on way street to absurdity.

@FrankHubeny What you're saying is "if I disregard the evidence for dark matter, then I don't know of any evidence for it". - the "theory explaining rotations of galaxies" is the evidence. The observed behaviour of galaxies is such that, by our understanding, there must be some sort of matter present that we cannot detect, or else our understanding of gravitation is incorrect. Occam's Razor is currently pointing towards the "mass we can't see" explanation, because of how many other observations seem to support our understanding. "Dark matter" is just a fancy way to say "matter we can't see".

"An absence of evidence is not evidence of existence"

@anaximander Galactic rotation is where the idea of dark matter started, but there's plenty of other evidence by now. We can sometimes "see" concentrations of mass by gravitational lensing, the Bullet Nebula being a particularly good example, and there are arguments related to the proportions of elements created in the early Universe along with observations of the mass of the known Universe.

@FrankHubeny Dark matter is also detected through gravitational lensing. This is outside of explaining rotations of galaxies, or even of purely gravitational theory, since the light is electromagnetic. They even made detailed maps of it based on this effect. But I do not see the analogy even if it was just rotations of galaxies. Something makes them rotate the way they do, "dark matter" just puts a label on it. In contrast, Santa and dragons are observationally idle fictions.

"Absence of evidence is not evidence of absence." - correct. "how are we supposed to conclude the potential non-existence of something" - you don't.

@Conifold It's the gravitational theory that requires dark matter not the actual measured rotations. Since there exist gravitational theories that don't require it (such as Moffat's gravitational theory) dark matter could be an idle fiction as well.

@anaximander When you wrote or else our understanding of gravitation is incorrect you expressed my point. Santa is an incorrect theory explaining how those presents appeared under the tree as predicted. Not finding dark matter would suggest we come up with a different explanation which is one way around the problem of not finding dark matter.

@FrankHubeny No, it could not. It may happen that some theory eventually replaces the current one (it will not be Moffat's, that is a relic from before lensing and other precision measurements). But within the current theory it is not observationally idle, and replacing it has no effect on that. Relativity eliminating ether did not make it observationally idle classically.

@FrankHubeny Moffat's gravitational theory fails to explain the Bullet Cluster, where we explicitly see gravitational lensing where there is no visible matter. The direct observation of lensing by invisible matter was basically the final nail in the coffin for all but the most convoluted large-scale alterations to gravity.

@probably_someone I'm not saying Moffat's theory is the best one out there nor that I agree with it. What I am saying is Einstein's gravitational theory needs dark matter and Moffat's doesn't. There exists a theory that doesn't require dark matter. Just as there exists an explanation for those presents under the tree besides the theory that Santa put them there.

@FrankHubeny The predictions of Moffat's theory are directly contradicted by the evidence the Bullet Cluster provides. It is not a valid explanation of reality. In a similar way, you could also say that "there exists a theory" of astronomy that doesn't require a round Earth, instead stipulating that the Earth is flat; this theory is not on the same footing as conventional astronomy, because flat-Earth theories are fairly clearly contradicted by evidence.

@FrankHubeny Not to speak for you but the logic for that dark matter must exist is this: we believe that we know how gravity works in distant regions of space that we cannot visit. However since our observations of those regions don't align with out predictions, there must be some invisible matter. Actually, since that doesn't actually make the observations work, we also have to assume dark energy exists because we know our current understand of gravity could not possibly be incomplete. The logic for this is similar to knowing that Mars must travel in epicycles because the it orbits the earth.

@JimmyJames "Actually, since that doesn't actually make the observations work, we also have to assume dark energy exists because we know our current understand of gravity could not possibly be incomplete." This isn't quite correct. The observations suggesting the existence of dark matter (anomalous gravitational lensing and galactic rotation curves) are quite different than the observations related to the existence of dark energy (the disconnect between the expansion rate of the universe and its energy density). You refer to them both as "the observations," which is misleading.

@probably_someone The separation you propose between the two concepts is again based on the assumption that our knowledge of 'laws' of gravity is complete. Or are you suggesting that the expansion rate of the universe has nothing to do with gravity?

@JimmyJames Also, we emphatically don't know that our current understanding of gravity couldn't possibly be incomplete, so I have no idea why you assert this. The reason we posit the existence of dark matter instead of modifying general relativity is because all but the most convoluted modifications to general relativity are contradicted by the evidence we've found, like the existence of an invisible galaxy-sized lump of mass in the Bullet Cluster that causes substantial gravitational lensing. As for the most convoluted modifications, Occam's Razor usually makes dark matter more plausible.

@probably_someone I didn't not it before but it's also the case that the most probably candidate for dark matter doesn't match with observation and that experiments that seem to have detect dark matter cannot be reliably reproduced. Note that I am not saying dark matter is not the correct hypothesis but rather that it is just that: a hypothesis. It's how sure people are about it that strikes me as relevant to the question.

@JimmyJames Can you provide some sources backing up your claims that "the most probably candidate for dark matter doesn't match with observation and that experiments that seem to have detect dark matter cannot be reliably reproduced"? This is totally at odds with what I have read. In any case, if what you call "the most probably candidate" doesn't match with observations, then it's not the most probable candidate anymore.

@probably_someone Occam's razor is simply a logic principle. It's not a scientific fact. There are many simple explanations that were held in physics that have to be discarded. Moreover because an alternate idea don't work doesn't mean dark energy exists.

@probably_someone "DAMA’s claim to have detected dark matter dates back to 1998 but has generated much controversy because similarly sensitive experiments have not confirmed DAMA’s findings."

@JimmyJames Maybe you have a different definition of Occam's Razor than is used in science. As far as I know, Occam's Razor states, "The explanation with the fewest assumptions that is not contradicted by any available evidence is likely to be the correct one." The "simple explanations" you're talking about actually violated Occam's Razor because they were contradicted by evidence.

@probably_someone "In any case, if what you call "the most probably candidate" doesn't match with observations, then it's not the most probable candidate anymore" There is no other more probable candidate so, yes, it's still the 'most probable'.

@JimmyJames "Moreover because an alternate idea don't work doesn't mean dark energy exists." I never said it did, and neither does anybody else in the scientific community. It's simply the best explanation we have given currently available evidence.

"that is not contradicted by any available evidence is likely to be the correct one." Likely being the key word. 'Likely true' is not the same 'is true'.

@JimmyJames :"There is no other more probable candidate" [Citation needed]

@JimmyJames If you are required to choose one of several different explanations, you should choose the one that is most likely to be correct. Do you disagree with this heuristic?

@JimmyJames DAMA is one experiment. There are dozens of other astrophysical experiments that provide corroborating evidence supporting the dark matter hypothesis that are unrelated to DAMA's results. These experiments have indeed been replicated; for example, the studies of the cosmic microwave background done by COBE and WMAP were replicated by the Planck satellite.

"It's simply the best explanation we have given currently available evidence" Exactly. Note how that differs from 'must be true' or 'is the case' which is how it is often discussed. Note the reaction in this thread itself to the notion that it might not be real. "If you are required to choose..." but we are not 'required' to choose an explanation. We can simply say, we have yet to find an answer.

@probably_someone Do yo think that there is an alternative to WIMPs that is widely considered more probable?

@probably_someone " There are dozens of other astrophysical experiments that provide corroborating evidence supporting the dark matter hypothesis that are unrelated to DAMA's results" [citation needed]

@JimmyJames No scientist will ever tell you that the existence of dark matter "must be true," because that's simply not how science works. Do you have proof that this has been discussed in this manner by reputable sources? The reaction in this thread is mainly because you seem to be misrepresenting the available evidence, not because of your opinion. And, if we want to progress in science, if we want to construct experiments to actually find out what's going on, we have to choose a hypothesis. If designed correctly, the experiment will provide evidence supporting or denying the hypothesis.

@probably_someone "No scientist will ever tell you that the existence of dark matter "must be true," because that's simply not how science works." So why are you arguing about this? What have I misrepresented?

@probably_someone This started with "I wonder if "dark matter" could be placed on the list with Santa and dragons.... there is no evidence for it that I am aware of." Are you saying that we know what dark matter is and that it has been directly observed or that there is no possible theory that could offer a simpler or more correct answer?

"No scientist will ever tell you that the existence of dark matter "must be true," Here's an article from phys.org that starts with this sentence: "Only a small part of the universe consists of visible matter. By far the largest part is invisible and consists of dark matter and dark energy." Would you not agree that this language presupposes the existence of dark matter? Maybe that site is junk, I wouldn't know but I see these types of statement frequently.

@JimmyJames, no true scientist!

35 comments? OK, we're done here. This is the dupe : Could there ever be evidence for an infinite being?

@FrankHubeny That is a kind of evidence, though. Why do you treat it as special?

@JimmyJames There are a few important things to know about science journalism: 1) The articles necessarily oversimplify scientific research. If they didn't, they would just end up being full research papers, and we have regular scientific journals for those. One of the most common oversimplifications is: any declarative statement like "X is Y" is a shorthand for "The hypothesis 'X is Y' is the most plausible explanation for our observations to date." You can see how inserting this language in every declarative statement would make science articles unreadable, hence the shorthand.

@JimmyJames 2) The articles are often not written by scientists; instead, science journalists are hired by these outlets to write the articles. These journalists often have backgrounds in science, but may not have experience doing scientific research. Since they're currently employed writing articles, they are definitely not currently doing research in the field they're writing about. As such, occasionally results are unintentionally oversimplified or even misinterpreted if something gets "lost in translation." If you want to hear what a scientist actually says, read their paper.

@JimmyJames Science journalists seem to assume that the reader is familiar with the basic philosophy of science, including the fact that declarative statements are necessarily expressions of the plausibility of a hypothesis after considering the available evidence. More transparency about this fact is probably warranted, as it's been the cause of many misunderstandings; that said, scientists themselves don't tend to worry about the basic philosophical details when talking to each other, and that tends to unfortunately translate into the journalism as a result.