NASA/ESA and The Hubble Heritage Team (STScI/AURA)
Why is dark matter assumed to exist in the cosmos? From reading news headlines you would think it has been clearly identified and that we now know so much about this once elusive stuff. It has been sought in many different laboratory experiments for more than four decades now, but never found. Why then are astronomers so confident it is out there? Let me try to put this into context and I hope it will become clear.
In my realm of interest there are really only two types of scientists:
Both construct mathematical models to describe their observations. Both test their models against those observations.
However the experimentalists (type 1) can interact with their experiments in a way the astrophysicists cannot. For example, they can send in a light signal and measure the response in the system, i.e. see what comes out. But the astrophysicists (type 2) cannot interact with what they are observing in the universe. The universe is just too large to do that.
Within our solar system we have been able to send probes to make observations. For example, NASA’s Deep Impact probe1 shot a 370 kg copper bullet into a comet2 and measured the spectra3 of the ejected material. And ESA’s Rosetta spacecraft landed a robotic lander, Philae, on a comet4 and made, for the first time, direct measurements of the surface constituents. These types of measurements, you could say, are very similar to what the experimentalists do in their laboratories. But the latter mission’s objectives, excerpted from the ESA website, highlight the type of science involved (emphases added):
There are basic underlying assumptions. The statement above makes it clear that the scientists who carried out the mission believe that the solar system evolved out of a solar nebula originating more than 4.6 billion years ago. That is the untestable primary assumption. It is not testable by what they dig out of the surface of the comet, but rather they believe the measurements of that material will help them understand the origin of the solar system within their original assumption.
But no matter how much evidence they accumulate they cannot directly observe the past. Certainly not without assumptions. They always need to apply interpretations to the evidence, which is the materials they dig out of those comets.
Even in the case of astrophysics, you might think that the astronomer is observing the past, because the light entering his telescope supposedly took millions or billions of years to traverse the vast universe to earth. But even this has its limits to what we can know.
The astronomer receives light into his telescope on earth and he must make the uniformitarian assumption that the light has been travelling at a constant speed (of about 300, 000 km/s) for the past millions or billions of years to reach earth, and with no relativistic time dilation effects. Only after making that assumption can he make the assumption, not know, that what he observes is coming from some past epoch millions or billions of years earlier. But how could you test that assumption? You can’t! And for that reason this aspect of astrophysics/cosmology is not directly provable by any empirical test.
In the case of all observations beyond the solar system the problem is beyond dispute. You cannot go there. The size, distances to and assumed age of galaxies, and other cosmic sources, is so great that even what we measure is as though we are taking a single still photograph; it is just a moment in time.
Astronomers only observe, they cannot interact with their experiment as the experimental physicist in the laboratory can do. And what makes matters even more difficult for the astrophysicist or cosmologist is that there are many possible explanations for the same observations. But because they cannot interact with the sources under investigation (which might even be the whole universe) their science is very weak indeed. For this very reason James Gunn, co-founder of the Sloan survey, said:
Now let’s examine this statement.
There exists a force of attraction between any two masses in the universe.
Can that statement be proven?
You might say, yes. We can locally test gravity and it works, in fact, it works extremely well, and it has been experimentally verified even down to sub-centimeter distances.7 Scientists find strong evidence in local laboratory experiments. In fact, new physics is even sought at distance scales less than this because it is believed that eventually the gravitational force law must break down, since quantum theory and Einstein’s gravitational theory are fundamentally incompatible. But all of those investigations are done via repeatable experimental physics. Different theories can be, and are being, tested by the experimentalists.
Evidence has accumulated that supports the law of gravitation. It has been tested repeatedly, and no contradiction found, to Einstein’s formulation of the law anyway. That is why it is now called a law. The law is often called the universal law of gravitation.
Can an experimentalist then safely extrapolate his conclusions about gravity, in a laboratory experiment, to the whole universe? No, he can’t, not without assumptions. Therein lies one major part of the problem.
Next we must decide on what we mean by ‘evidence’. Normally evidence is data collected. But that data must be interpreted. And models are constructed that make predictions. In a laboratory the experimentalist can test those predictions. In the cosmos this is more difficult. It is possible, though. A model might predict the existence of certain behaviour and then the astrophysicist looks for that. But it is more like ‘stamp-collecting’ than laboratory science. Because he cannot do an experiment he accumulates as many observations as possible and tries to classify the results. He sorts his objects into families, or identifies a common trend among those in the same family. By accumulating a lot of such data he argues for his model. But because he cannot know by an empirical test his conclusions are invariably and inevitably weak.
Now, what if you were to read the headline, “Evidence for dark matter in the inner Milky Way”?8 What evidence could this be referring to? And how could you know that dark matter really exists?
The 2015 article that bore this headline went on to say (emphases added):
Without going into the details of the physics of gravitation, and why bodies like stars orbit the Galaxy centre the way they do, we can learn by critically examining these statements.
Dark matter is called dark because we cannot see it. No dark matter particle has ever been observed in a lab experiment despite more than 40 years of searching. I even spent a few years doing that myself looking for paraphotons, which are classed as WISPs,10 a putative dark sector particle. (In Star Wars terminology it is a particle from the Dark Side.)
If the so-called dark matter particles could be observed by light, or by X-rays, or by some other electromagnetic radiation, it would make their identification easy. But how can the author of the above article claim “direct observational proof”? How can it be claimed that their existence is “well established” in the outer parts of our galaxy? They don’t observe dark matter, nor do they do an experiment where they send in some radiation into a cloud and get a response back.
An experimentalist might do something like that to detect a particle he cannot otherwise ‘see’. So the act of seeing means a response to some radiation. It does not mean a human has to be able to see it with his own eyes. For example, we know electrons exist. That is not in dispute, and their existence has been repeatedly confirmed by many experiments. (Interestingly, though, we don’t know how small they are. It is still an open question. But I digress.)
In the galaxy, how can the claim be made that the observations cannot be explained unless large amounts of unseen dark matter are assumed? To make such a claim, you would have to know that you have ruled out all other possibilities. In such a case—remember this is not a laboratory experiment—you would have to be an all-knowing god.
Background photo: Serge Brunier/NASA
The image displays the rotation curve tracer gases from the publication11 over a photograph of the disc of the Milky Way Galaxy as seen from the Southern Hemisphere. The tracers are colour-coded in blue or red according to their relative Doppler motion with respect to the Sun. The spherically symmetric blue halo illustrates the dark matter distribution inferred from the analysis.
The light coming from the gases and stars in the Galaxy is observed with a telescope, but more specifically what is observed are the spectral lines in the light from those sources. And they are seen to be red-shifted or blue-shifted. (Spectral lines shifted toward either the red end or the blue end of the spectrum, as compared to a laboratory sample of the same type of gas.) These effects are interpreted as arising from the well-established Doppler Effect, where the motion of the gas particles (or the stars) causes this effect in the light. Then that is interpreted as meaning the gases and stars are moving around the galaxy centre at certain speeds, which are typically 100 km/s to 300 km/s.
That interpretation (the Doppler Effect) requires some assumptions. But all are reasonable and within known laboratory physics, except one. That one is that the law of gravitation is true out in the Galaxy where these gases or stars are. That is, the law of gravitation, which has been tested extremely well in the solar system, also applies without modification out in the cosmos, both in the Milky Way Galaxy and outside it.
But we have already realized that it cannot be known if that law is universal. It is assumed to be universal and thus a model of the galaxy constructed. If the observed speeds (and they are an interpretation of the meaning of the red-shifted and blue-shifted light, which we will agree to here) follow the expected trend then we say all is well and Newton’s law (of gravity) works fine. But the problem is they do not. The stars and gases move too fast around the Galaxy to obey Newton’s law. If that situation continued for hundreds of millions of years the stars in the Galaxy disk region (where our solar system in located) would fly apart and the Galaxy would disintegrate over longer time scales. But that couldn’t be right because (of the underlying assumption that) the galaxy is stable and has been around for 10 billion years or so.
So the conclusion is, either that the law of gravitation is wrong or that there is more matter in the Galaxy, which we cannot see. Nearly always it is assumed that there is missing, hence ‘dark’, matter lurking out there, which comprises 80–90% of a galaxy’s mass.12 But if it wasn’t for Einstein’s discovery where he added to and improved on Newton’s law of gravitation, we might still be thinking that the dark planet Vulcan (also called dark matter at the time) was necessary to explain an anomaly of Mercury’s orbit in the inner solar system.13 So it is just as reasonable to think that new physics rather than new matter is needed to add to Einstein’s improvements the way these did to Newtonian physics. Yet nowadays news headlines tend to speak of discoveries of dark matter as though it is being directly imaged.
“Dark matter observed in the heart of our galaxy,”14 says one news headline, and the article states, “But up to now it has proven very difficult to establish the presence of dark matter in the innermost regions.”15 You get the impression it is all now well-established science. This is illustrated in the figure above of the Milky Way galaxy with the red-shifted and blue-shifted sources shown on either side of the central core of the Galaxy.
Another headline: “Milky Way has half the amount of dark matter as previously thought, new measurements reveal”.16 The story is that by looking at how many dwarf satellite galaxies our galaxy has around it, and their motions, you can determine the mass of our galaxy. This is important because according to the standard cosmogony the mass of the Galaxy determines its formation process, and that process is determined from the cosmology that is assumed.
In this story these so-called measurements solve a mystery. Remember the big bang model is assumed. That is called the Lambda (Dark Energy) Cold Dark Matter theory, which predicts that there should be several big satellite galaxies around our Milky Way Galaxy that are visible to the naked eye. But that is not what we observe. However, the new measurements supposedly solve the problem (emphases added):
Firstly, you cannot predict something that you know prior to formulating your theory. It is not a prediction. Secondly, the dark matter also cannot be observed. The ‘amount’ is derived from the motion of the dwarf galaxies and/or the stars and gases in our galaxy, but that assumes that ‘missing matter’ is the reason for the anomalous motion.
From the news report you would think that dark matter is directly observed; but it is not. But why is it so important? Just like the Rosetta space probe was to discover the origin of the Solar System, the ‘dark’ matter mapping is to discover the evolution of the Galaxy (emphases added):
Dark matter, though never identified in a lab experiment, is assumed a priori.
The same type of analysis is applied not only to galaxies, satellite galaxies, galaxy clusters, and super-clusters but also the whole universe.
“Too much dark matter in galaxy cluster? ‘Dark core’ may not be so dark after all”19 reads another headline. When clusters are analysed it was still via the assumed ‘motions’ or properties of gases in the clusters, or constituent galaxies, in some cases. Never is dark matter observed, only the inferred motion of ‘particles’ under the assumption of the universal law of gravitation.
Due to application of well-established laws, including gravitation, with the assumption that the galaxy clusters are stable over their assumed many-billions-of-year lifetimes, it is determined in this case there is a lack of dark matter over what was expected. This all comes about because of the application of the law of gravitation to these super-massive objects, and uniformitarian interpretations applied. None of which can be proven.
In this research, on observations of the Abell 520 galaxy cluster, something that came out of Einstein’s improved theory on gravitation is used. This was not found in Newton’s theory. It is gravitational lensing, where, according to the theory, the matter of cluster(s) warps the path of light through space and can be thought of as a giant lens. By modelling the gravitational lens using dark matter they try to match theory to observations and hence ‘infer the presence of dark matter’ in the cluster. They do not claim ‘direct’ imaging of the dark matter.
This essentially becomes circular reasoning. It proceeds like this; the universe is stable and has evolved over more than 10 billion years producing galaxies and galaxy clusters. The only agency doing the ‘creating’ of galaxies and clusters was/is the law of gravitation acting on the matter. That is the primary assumption, in the background of the assumed cosmology, which also includes the weirdest stuff of all, Dark Energy. Then to make the observations fit the theoretical model, dark matter must be included, else the model should be rejected. So the ‘existence’ of dark matter is the product of the initial underlying uniformitarian assumption. Because the universe itself supposedly constructed galaxies and clusters from only matter under the influence of gravitation, it follows that there must be an enormous content of invisible matter that cannot be seen.
Alternatively if you assumed the galaxies were not that old, and/or they were not in a stable state because they have not existed for billions of years, you would not need to include any dark matter. Or even if they have existed for at least hundreds of millions of years based on their spiral structure (and some biblical creationist time dilation cosmologies allow for that, while only a short time passes in reality, if earth clocks are the reference frame) another possibility is that they are stable and there is a need for new physics, an extension of the law of gravitation that applies on very large scales in the universe.
These ideas can be consistent with a straightforward interpretation of Genesis that states clearly the universe was created approximately 6000 years ago (as measured by earth clocks). So why resort to dark matter? Because ultimately it is so one can believe that the universe naturalistically created itself and there is no Creator God.
But some may claim science has detected radiation from dark matter particles in the cosmos. I previously reported on the idea that the intergalactic medium has too much light coming from it, where no sources could be identified, and it was theorized as the result of the decay of some hypothetical dark matter particles. This resulted from a mismatch between theory and observations, and hence dark matter was suggested as the solution.
By now you must have realized how convenient it is to have dark matter particles, or in fact anything from the dark sector. It can fill in what is missing in the theory without a need to reject the underlying theory itself.
Apparently two groups of astronomers have found a signal among some X-rays coming from different galaxy clusters. It cannot be explained by known physics, so dark matter is concluded.
I have previously discussed the hypothetical sterile neutrino, which is sometimes called Dark Radiation, proffered to rescue the Standard Model of particle physics, when the standard big bang cosmology is assumed to be true. The problem arises essentially only in cosmology and astrophysics because the very successful theory of particle physics, which has been extremely well tested in repeatable laboratory experiments, has no real need for another neutrino.
What is the goal of looking for dark matter in the cosmos? One of the scientists in the X-ray study, Boyarsky, says:
Need I say more? It is philosophically driven. Materialistic naturalism is all that is assumed and dark matter is the god which fills in the gaps to maintain the façade.
Dark matter is crucial in the formation of both stars and galaxies. Without it they won’t form naturalistically. I will deal with star formation is another article, but consider this; If you don’t know how stars and galaxies formed, you don’t know much about how the universe, which we observe, formed.
Galaxy formation is a seriously big problem for big bang cosmology. In the computer simulations, modelling the formation of the large-scale structure (super-clusters, filaments of galaxies etc) in the universe, dark matter is assumed from the beginning. For the condensation of individual galaxies it is a similar story. By starting with a critical density of dark matter the models are able to show galaxy formation under gravity where the dark matter attracts the normal matter into the central region to form a galaxy.
The dark matter must reside as a spherical halo around the spiral galaxies, which have a thin disk of luminous normal matter. This state is determined from the studies of the speeds of gases and stars in the disks of thousands of spiral galaxies. But there is even a problem here too, called the dark matter cusp problem.
The problem is because the unseen made-up stuff does not quite behave as you would expect matter to behave under the influence of the law of gravitation.
Since dark matter is meant to be like normal matter under gravity’s influence it should pile up in the centre of galaxies. Its density should be maximum in the core—hence, there should be a cusp or peak in the density distribution there. But to accurately model the motions of the stars and gases, dark matter is not needed in the central cores, only in the disk regions. Newtonian gravity alone easily accounts for the visible matter in the central nuclei of these galaxies.
NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
So even the studies that infer the existence of the dark matter contradict what matter should do under the influence of gravity.
One study on dwarf galaxies highlights this problem (emphases added):
‘Pitless peaches’ means no dark matter in the centre of these galaxies, just where ‘gravity says’ it should be found. Even though these researchers have implicitly assumed that 99% of the matter content of those galaxies is dark matter, their own observations (of motions of the stars and gases) do not agree with the dark matter paradigm. To my knowledge this is true in all galaxies where such studies have been done.
Astrophysics and cosmology are by their very nature loaded with philosophical underpinnings. In principle there is nothing wrong with that. You could not do any sort of science without a basis to build your model. I would call these philosophies worldviews. And we all have a worldview. We form that based on what we believe about the world around us and how it all began. The difference here is that my worldview is based on the biblical truth that God, the Creator, created the universe about 6000 years ago. It was not the result of an accident or a quantum fluctuation of some false vacuum or a big bang of any sort. If it were, God would have said so in the Bible.
The worldview that underlies modern cosmology, and cosmogony (on the origin of the universe) is an atheistic one. It has no place for a Creator, and only relies on what man can discover for himself. As a result he has had to resort to all sorts of fudge factors to make his model fit the observational data, the evidence from the cosmos. Dark matter has arisen from this. But even when assumed to fix such problems the supposed dark matter does not behave like normal matter under the influence of gravity. It is stranger than fiction and I am afraid it is no more real than the ‘Emperors’ new clothes’.