This week scientists announced the result of a three-year search for dark matter using the most sensitive instruments. The result: a big fat nothing.
“We built an experiment that has delivered world-leading sensitivity in multiple new results over the last three years,” said a scientist. “We gave dark matter every opportunity to show up in our experiment, but it chose not to.”
Why, then, are scientists continuing to be so dogmatic about dark matter? The first way in which scientists are dogmatic about dark matter is the way in which they assert its existence unconditionally. Matter-of-fact assertions that something exists should not be made by scientists unless there is direction observational evidence. When asked what we know about dark matter, a typical physicist will answer that we know it exists. No, we don't. There are some astronomical observations that dark matter might be convenient in explaining, but such a situation does not justify matter-of-fact assertions that dark matter exists. Dark matter may be one way of explaining puzzling astronomical observations, but there are always many different ways of explaining a puzzling observation.
The second way in which scientists are dogmatic about dark matter is in the way they claim we know what percent of the universe is dark matter. We hear scientists making claims such as the universe is 26.8% dark matter and 68.3% dark energy. Given that neither dark matter nor dark energy has been directly observed, such claims are rather like claiming that the population of heaven is 23% angels and 12% archangels.
If you do a Google search for “evidence for dark matter,” you will find various web sites that argue for the existence of dark matter. But the reasons given are not very compelling. One common reason goes like this: scientists have compared what is called the gravitational mass of galaxies with the mass inferred from the luminosity of stars. It seems that these two figures do not match with each other, and that dark matter might resolve the discrepancy. This is not a very convincing reason, because it is all too possible that scientists may have underestimated or overestimated either of these two things. It is also all too possible that various non-luminous forms of regular matter might resolve this discrepancy – things such as dust, brown dwarfs and black holes (all of which are either hard to observe or hard to quantify). Since we are faced with at least 1000% uncertainty when estimating the total mass contribution from dust, brown dwarfs and black holes, we can't reliably calculate the total amount of mass from regular matter in the galaxy.
Another reason given for believing in dark matter has to do with what are called galactic rotation curves. Without any assumption of dark matter, scientists expect stars around the outer edges of our galaxy to be rotating the galaxy more slowly than stars closer to the center of our galaxy. But instead they found that the velocity of stars not close to the galactic center remained roughly constant, regardless of how far they are from the galactic center.
There is a straightforward way to interpret such observations. We can assume that there is some principle that is causing stars to rotate the galaxy at the same speed, perhaps some principle that is related to the fact that if stars did not rotate the galaxy at the same speed, spiral galaxies would not preserve their structure, and we would no longer have our beautiful spiral galaxies. But instead of evoking such a principle, astronomers advance the weird idea that there is a large outer halo of invisible dark matter, which happens to be arranged in such a way so that (just coincidentally) stars rotate the galaxy at the same speed. Now if astronomers wish to advance such a contrived explanation, they may do so. But such an explanation (which is far from straightforward) is only one of many ways of explaining the fact that stars rotate galaxies at the same speed. It is not at all correct to cite such a fact as strong evidence for dark matter, although it may be evidence that there is something out there interested in preserving the universe's beautiful spiral galaxies (such a power might be a transcendent power, or perhaps something like billion-year-old extraterrestrials with godlike powers).
Web sites arguing for the existence of dark matter also tend to cite the cosmic background radiation. We will be shown some picture of the cosmic background radiation, one of those maps that exaggerates the differences in this radiation that is uniform to one part in 10,000. We are told that the darker spots on the map are “dark matter concentrations.” But, in fact, the cosmic background radiation provides no support at all for the concept of dark matter. A realistic map of the cosmic background radiation will use only one color, since the radiation is uniform to one part in 10,000.
Some writers also claim that something called the Bullet Cluster provides evidence for dark matter. These claims are based on a statement of Doug Clowe of the University of Arizona, who in 2006 did a study on the Bullet Cluster. “"These results are direct proof that dark matter exists,” Clowe assured us. But his results were no such thing. Certainly not direct proof, his results were neither proof nor compelling evidence for dark matter.
Clowe merely studied a funny-looked cluster, and provided a complicated dark matter story as an explanation. But there are always 101 ways to explain some strange-looking astronomical object. Some funny-looking cluster that might be explained by dark matter is not proof for dark matter. See here for an explanation of the Bullet Cluster that does not involve dark matter.
Some gullible NASA personnel uncritically quoted Clowe's unwarranted claims. People were fooled by a caption on a visual showing the Bullet Cluster. The visual showed blue areas and pink areas. Captions to the visual told us that the blue areas were dark matter and the pink areas were regular matter.
The Bullet Cluster (which doesn't look like this to the eye)
You can realize here the misleading silliness when you remember that dark matter is not believed to be blue, and is not even believed to be visible. The image in question was a composite image, made from combining one image from one regular telescope and one image from a radio telescope. The blue areas were simply areas emitting different amounts of radiation. The claim that the blue areas were dark matter is an unproven speculation. There is a rather similar cluster called the Train Wreck cluster which is very hard to explain under dark matter assumptions.
The central triumph of modern physics is what is known as the Standard Model of physics. Dark matter has no place in that model.
When dealing with dark matter, our scientists are plagued by what is known as confirmation bias. When confirmation bias occurs, someone may eagerly scout for anything that might be interpreted as evidence for something that he wants to believe in, but also ignore any evidence that is inconsistent with the thing he wants to believe in. The scientific paper here presents quite a bit of evidence that is inconsistent with dark matter assumptions.
The paper above quotes Karl Popper giving this description of confirmation bias:
For if we are uncritical we shall always find what we want: we shall look for, and find, confirmations, and we shall look away from, and not see, whatever might be dangerous to our pet theories. In this way it is only too easy to obtain what appears to be overwhelming evidence in favor of a theory which, if approached critically, would have been refuted.
I am not claiming that the existence of dark matter is very unlikely. I merely claim that nothing we have learned warrants dogmatic assertions claiming that dark matter definitely exists. Something unproven should not be represented as something proven, and speculations should be candidly described as speculations.