A large part of the universe is lost.
It’s nowhere to be seen, at least nothing that humanity can technologically capture at this point. But astronomers know there is something out there—the gravitational pull of the invisible material that makes up about 85% of the universe’s mass. Currently, the best explanation scientists have for this is dark matter, a hypothetical form of matter that does not absorb, emit, or reflect light.
Accounting for dark matter solves many unexplained cosmological phenomenaand this ease means that most astronomers readily accept that dark matter exists. Accordingly, many top institutions devotes worldwide resources to carefully designed, technologically unusual experiments to detect dark matter.
All that said, we have yet to find irrefutable evidence of dark matter. Now, it’s worth emphasizing again that, as you can see, the indirect and theoretical evidence is overwhelmingly in favor of dark matter. But in physics, there’s always the small possibility that we’re completely off the mark, and nature has other plans for how reality works.
So, in this Secret Survey, we asked the experts to at least consider the question – what if dark matter doesn’t actually exist? What makes us so sure that dark matter is the right answer? And even so, if it is physically impossible for humanity to find dark matter? All things considered, what are the alternatives—however “fringe”—to explain the missing mass of the universe?
The answers below may have been edited and shortened for clarity.
Vedant Chandra
astrophysicist, Center for Astrophysics | Harvard and the Smithsonian.
Disproving the existence of dark matter is a tall order. One reason is that so little is known about it—reasonable models span tens of orders of magnitude in mass. Although various experiments have made progress ruling out parts of this parameter space, the untested candidate space is vast. From an astrophysical point of view, some matter behaving like dark matter remains the simplest explanation for all our observations to date.
If dark matter doesn’t really exist, a significantly more complex change to physics would be needed to explain all of our observations simultaneously. Looking ahead, leading dark matter models predict its clustering down to sub-galactic scales, filling galaxies with a completely invisible dark matter substructure. These structures can be detected by lensing or by their gravitational influence on the thin stellar streams orbiting the Milky Way. Future detections of this dark substructure will be further confirmation of dark matter acting independently of any visible matter.
Sabine Hossenfelder
Physics and science communicator, Science with Sabine.
If dark matter does not exist, the observations that lead astrophysicists to the conclusion that it exists will not disappear. So we’ll need something else to explain the observations. Currently, the most plausible alternative explanation is that we have misunderstood the law of attraction. Personally, I would find this more interesting, as it could be a transition to quantum gravity and possibly explain dark energy.
That is, “dark matter” is so vaguely defined that it cannot be falsified. At best, we can “note” it by finding a better explanation. Right now though altered gravity equally good or bad as dark matter; each has its advantages and disadvantages. Personally, I find both explanations unsatisfactory. I would really like to see this problem attacked by AI.
Stacy McGaugh
cosmologist, Case Western Reserve University; is the author of the blog Triton station.
Astronomical observations have established beyond any doubt that there is a discrepancy between what we see and what we get in extragalactic systems—galaxies, galaxy clusters, and the universe as a whole. When we apply the law of gravity taught to us by Newton and Einstein to the matter we can see in these systems, we arrive at a short conclusion: the observed motions mean more mass than is apparent: dark matter.
Dark matter is based on the assumption that Newton and Einstein taught us everything we need to know about gravity. This seems like a good assumption, but it is only an assumption. If dark matter does not actually exist, this implies that there is much more to learn about gravity. The observed discrepancies may be due to force law variation rather than invisible mass.
One proposed modification to the power law is a theory known as MOND (Modified Newtonian Dynamics). MOND made numerous predictions that were later confirmed by observation. This shouldn’t happen in a dark matter universe, and it should make us wonder: Is the invisible mass in the room with us now?
Juan I. Collar
astrophysicist, Kavli Institute for Cosmological Physics, University of Chicago.
Shortly after Maxwell defined light as a traveling electromagnetic wave, experimentalists began to measure the presence of the medium through which it propagates, the luminous aether, or aether for short. Although Maxwell’s equations provided a recipe for self-propagating electric and magnetic fields that were never based on the aether, the bias against its existence was overwhelming. No evidence was found until nearly twenty years later when the theory of relativity finally put it to rest. In retrospect, there was no explanation for our ethereal obsession other than pure, unrefined human bias with a slight lack of imagination.
Particle searches for dark matter have taken at least twice as long as ethereal searches, with a persistence of negative results to show for our efforts. The gravitational effects of what we call “dark matter” are indisputable. To conclude that there is a new fundamental particle behind them, or that the particle in question interacts through some mechanism other than gravity, is to retrace the obsessive-compulsive steps of experimentalists after Maxwell’s realization.
Short of a new Einstein to put us out of our misery, a solution (compassionate at this point) may come from observational astrophysics. Zero or no information about the nature of dark matter particles (constraints on mass, type, and combination) has been provided to the experimental community searching for them from the field. Emphasis on providing this guidance could direct the current begrudging disarray of multi-particle opportunity searches to efforts with a higher chance of success.
Kyu-Hyun Chae
physicist, Sejong UniversitySouth Korea.
The concept of “dark matter” has certain similarities with historical concepts such as “epicycles” or “aether”, as they are logical consequences of well-established paradigms/and theories. Eventually, epicycles and ether were disproved and led to major scientific revolutions. Dark matter is a logical consequence of standard gravity according to Newton and Einstein. The dark matter paradigm has been the standard view for nearly a century since Fritz Zwicky and other astronomers observed gravitational anomalies in astronomical systems such as galaxy clusters and galaxies in the 1930s and beyond. The apparent success of dark matter in explaining astronomical data may have some similarities to the apparent success of the epicycle-based Ptolemaic geocentric model in explaining the observed planetary motions in the sky.
Since epicycles ultimately do not survive smoking gun evidence such as Galileo’s observation of the phases of Venus, it is likely that astronomical smoking gun evidence will eventually tell whether dark matter is another epicycle/ether or a true physical entity. Such smoking gun evidence can be obtained from observations that show direct discrepancies with standard weight. Scientific articles provides this evidence has been published in recent years. In particular, detailed observations the internal dynamics of large binary stars and galactic rotation curves will provide crucial evidence in the coming years. I think a new scientific revolution is underway (probably) and will be welcomed by many scientists. It is perhaps ironic that history consistently shows that scientific revolutions occur at the expense of seemingly perfect physical theories. These are exciting times in gravity and cosmology!
Don Lincoln
great scientist Fermilab; science communicator.
The debate over whether dark matter or altered physics holds the answer to cosmic mysteries like rapidly spinning galaxies is both fascinating and inspiring, and it’s a fascinating topic for physicists. Over the years, I’ve gravitated toward “modified physics”; However, two observations put me firmly in the dark matter camp. The Bullet Cluster strongly supports the dark matter hypothesis, as does the observation of ultra-diffuse galaxies in the NGC-1052 group. These galaxies, called DF2 and DF4, appear to be governed by currently accepted laws of physics. The existence of galaxies that appear to contain no dark matter at the moment of cosmic irony is strong evidence for the existence of dark matter. If the answer to the question was altered physics, there would be no rotating galaxies as predicted by Newton’s laws.
Assuming that particulate dark matter is real, the question is “what is its nature?” The answer is unavailable. Sensitive dark matter candidates have been hypothesized with masses ranging from 10 to 11 times the mass of an electron to the mass of a medium-sized asteroid. Experiments have ruled out classes of possible dark matter candidates with specific properties between 1 and 1,000 times the mass of a proton, although many possible candidates remain viable. Whether or not we can detect dark matter particles directly depends on how they interact. If they turn out to interact only gravitationally, we’ll never be able to detect them. If we ever hope to know the answer, the only way forward is to continue to search for dark matter in as many creative ways as possible. Hopefully, one lucky group of researchers will make an observation that answers this very interesting question.
Indranil Banik
astrophysicist, University of Portsmouth.
Of course, not all astronomers are happy about dark matter. To settle for just the apparent mass, we have to relate their apparent matter to the observed gravitational force, which comes from how fast the galaxies are spinning. I have believed in MOND for ten years, during which time I have worked to test MOND in various ways. By conducting a detailed analysis of Gaia observations of thousands of large binaries in the solar neighborhood, I was able to do this. strongly refutes the MOND prediction.
All of this suggests that you need dark matter, even before considering the evidence from cosmology. I recently published it paper On galaxy formation without dark matter, which is required to act as a seed to form around a galaxy in the Standard Picture. If you consider outer solar system planets like Saturn, large binary stars in the solar neighborhood, or the fact that galaxies formed extremely quickly after the Big Bang, the observations are better explained using standard gravity combined with dark matter. I didn’t come to this conclusion by staying in my comfort zone, because I’ve believed in MOND for ten years, and in that time I’ve published many papers and a 150-page long study defending it. But I now believe that if there is anything wrong with our theory of gravity, the inaccuracy is very small until we reach a distance of at least 100 million light years. On a larger scale, there are reasons to think that our theories don’t work well. However, changes to gravity at these scales cannot replace the need for dark matter.
Therefore, my answer to the question posed here is that if dark matter does not actually exist, even if it is willing to modify gravity, one can make very little sense of the Universe, since the universe has very little flexibility to hope to do so with only visible matter. In short, I believe that dark matter does exist and is a major component of galaxies and larger structures such as galaxy clusters.
