The theory of dark matter has struck out at the plate again. Scientists reported that they have observed a galaxy named NGC1052-DF2 that seems to have no dark matter near it. This contradicts the dogmatic claim of dark matter theorists that there is a halo of dark matter surrounding every galaxy.
This is the second blow this year against the dark matter theory. It was only last month that we had headlines such as “New Observations of Galaxies Challenge the Standard Cosmological Model.” It was found that 14 out of 17 satellite galaxies orbit the galaxy Centaurus A in a flat plane-like orbit, not randomly scattered in a sphere surrounding that galaxy, as predicted by dark matter theorists. The same type of situation had previously been found in regard to our own Milky Way galaxy and the large nearby galaxy Andromeda. In all three of these cases, satellite galaxies are positioned in roughly a disk-like shape, rather than scattered in a sphere-like shape as predicted by dark matter theory.
Centaurus A (credit: NASA)
The headlines were based on a scientific paper that estimates that the chance of finding a plane-like arrangement of satellite galaxies is only 1 in 200, and which says that the chance of finding three galaxies with such an arrangement is “extremely unlikely” under dark matter assumptions.
The latest discovery concerning NGC1052-DF2 has prompted some nonsensical headlines. Forbes.com has a story entitled, “Bizarre Ghost Galaxy Has Hardly Any Dark Matter - Proving That Dark Matter Exists.” It quotes a Yale professor Pieter van Dokkum as saying the following:
We thought that every galaxy had dark matter and that dark matter is how a galaxy begins. This invisible, mysterious substance is the most dominant aspect of any galaxy, so finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. It has its own separate existence apart from other components of galaxies.
How's that professor? Finding a galaxy that you don't think has any dark matter shows that dark matter is real? That's defective reasoning, like someone arguing that the fact that he didn't see any unicorns today just proves that unicorns must exist.
We get another look into the logic of van Dokkum in an article on Quanta.org, with the misleading headline, “A Victory for Dark Matter in a Galaxy Without Any.” In that article van Dokkum reasons that other theories would also have a hard time explaining this NGC1052-DF2 galaxy. But an adherent of the dark matter theory is not a reliable source on what competing theories do or do not explain or predict. Scientists in general who become fanboys of some particular theory tend to know very little about competing theories, and often have distorted or jaundiced views about competing theories. Also, it makes no sense to argue that your theory is right because something was observed that conflicts with both your theory and a rival theory. In such a case, the most likely thing is that neither of the theories is correct.
It is, of course, a complete fallacy to be assuming that one of the theories about some natural topic must be true. We may have Theory A, Theory B and Theory C to explain Topic X, but there is no reason to assume that one of these theories must be true. The correct explanation might be Theory D or Theory E or Theory F, none of which humans have ever considered.
We also get a misleading headline from Nature.com, which has a story entitled, “Beguiling dark matter signal persists 20 years on.” That sounds like dark matter has been detected. But the text of the article tells a different story. We hear that “many physicists still express skepticism” about this signal, and some reasons for thinking it's not dark matter. So in that case, why does the story's headline refer to a “dark matter signal” ?
Nowadays there is a situation where any astronomer who sees something baffling he can't explain may tend to call it “a possible sign of dark matter.” But there have been no reliable observations directly showing dark matter exists, and expensive projects trying to detect it directly have failed. Nor do we have any theoretical understanding of dark matter on the particle physics level. The theory at the center of particle physics is called the Standard Model of Physics. Dark matter has no place in such a theory.
So why do astronomers go about claiming that this or that galaxy has dark matter near it? They make such claims whenever they see galaxies behaving in surprising ways they can't explain through ordinary gravity produced by regular matter.
What led to the belief in dark matter was the discrepancy shown in the visual below. Astronomers thought that the rotation velocity of stars (the speed at which they rotate around the center of the galaxy) should decrease the more the stars are located from the center of a galaxy (which would be the behavior shown by the blue line below). But instead stars rotated with the speed shown in the red line.
Such a discrepancy was certainly not anything that directly suggested that dark matter existed. It was merely a case of nature behaving in a surprising way. Scientists tried to explain this discrepancy by advancing a very contrived, ad-hoc, and speculative assumption: that each galaxy was surrounded by a kind of envelope or halo of invisible dark matter. Such a theory involves two assumptions: the assumption of the existence of such invisible matter, and also a very specific assumption about the arrangement and placement of such matter. The astronomer making such an assumption is like some theologian confidently telling you not merely that angels exist, but that they live on top of clouds where we can't see them (which would involve not just an assumption about an unseen, but a very specific assumption about the position of such an unseen).
The “overwhelming evidence for dark matter” cited by astronomers is no such thing. It's just evidence that stars rotate with speeds that have a surprising uniformity that we don't understand. When an astronomer says that galaxy X has dark matter, he essentially is just saying that the stars revolve around the center of a galaxy with the pattern shown in the graph above. That isn't really something that tells us dark matter exists, but merely a hint that it might exist. A science paper found (as discussed here) that the rotation speed of galaxies is well-correlated with the amount of visible matter, something that makes no sense under the theory of dark matter.
Showing their love for being obscure in unnecessary ways, scientists use “^CDM” to signify the cold dark matter theory, in which the first character is the Greek letter lambda. Such a phrase can be expressed as Lambda Cold Dark Matter. But that Lambda word tells you nothing. The theory should be called the Specially Placed Invisible Matter theory or SPIM. That would remind us that the theory relies not merely on postulating invisible matter, but on special assumptions about the way such matter is placed.
To help shed light on whether a claim is warranted, it is sometimes a good idea to put scientific reasoning in kind of a syllogistic nutshell. The Big Bang theory holds up pretty well to such a thing. We can use reasoning like this:
Premise 1: Astronomers know from red shifts that all the galaxies are expanding away from each other.
Premise 2: The universe has a type of background radiation that we would expect it to have if the universe was once in a very dense state.
Conclusion: The universe must have started expanding from a state of incredible density.
That conclusion holds up reasonably well. But let's try the same thing with dark matter.
Premise 1: The stars in a galaxy rotate around the center of the galaxy at a much more uniform speed than we would expect if just visible matter is involved, given our current understanding of gravity.
Conclusion: Therefore, such a galaxy must be surrounded by an invisible halo of dark matter, some substance never directly observed, with this halo arranged in a very particular way.
This conclusion does not at all follow from the premise. It is merely a speculation, one of many possible ways in which the rotation discrepancy might be explained, including new laws of nature.
Scientists who talk in a matter-of-fact way about dark matter are merely another example of something we see all too commonly: scientists acting as if they know things that they do not actually know. The problem is that scientists fall in love with their speculative theories, forgetting they are conjectural, and a scientist may "live-breath-and-eat" his favorite theory like some novelist totally wrapped up in his novel or like some ardent suitor obsessed with his loved one.