A huge chunk of the universe is missing.
It’s nowhere to be seen, at least not with anything that humanity is technologically capable of capturing at this moment. But astronomers know that there’s something there—the gravitational influence of an invisible material that constitutes roughly 85% of the universe’s mass. As of now, the best explanation scientists have for this is dark matter, a hypothetical form of matter that doesn’t absorb, emit, or reflect light.
Accounting for dark matter solves a lot of unexplained cosmological phenomena, and this convenience means that most astronomers readily accept that dark matter exists. Accordingly, many top-notch institutions worldwide dedicate resources toward carefully designed, technologically extraordinary experiments for detecting dark matter.
All that said, we’ve yet to find irrefutable proof of dark matter. Now, it’s worth emphasizing again that, as you’ll see, the indirect and theoretical evidence largely rules in favor of dark matter. But in physics, there’s always the slim possibility that we’re completely off the mark, and nature had other plans for how reality works.
So, in this Giz Asks, we asked experts to at least consider the question—what if dark matter doesn’t actually exist? What makes us so certain that dark matter is the right answer? And even if it is, what if it’s physically impossible for humanity to find dark matter? All things considered, what are the alternatives—however “fringe” they may be—to explain the missing mass of the universe?
The following responses may have been edited and condensed for clarity.
Vedant Chandra
Astrophysicist, Center for Astrophysics | Harvard & Smithsonian.
Disproving the existence of dark matter is a tall order. One reason is precisely because so little is known about it—plausible models span dozens of orders of magnitude in mass. Although various experiments have made progress ruling out portions of this parameter space, the untested candidate space is vast. From the perspective of astrophysics, some substance that behaves like dark matter remains the simplest explanation for all our observations to date.
If dark matter truly doesn’t exist, some substantially more complicated alteration to physics is required in order to simultaneously explain all of our observations. Looking ahead, leading dark matter models predict its clustering down to sub-galactic scales, filling galaxies with dark matter substructure that is entirely invisible. These structures can be gravitationally detected through lensing or by their influence on thin stellar streams that orbit the Milky Way. Future detections of this dark substructure would be yet another confirmation of dark matter that operates independently of any visible matter.
Sabine Hossenfelder
Physicist and science communicator, Science With Sabine.
If dark matter doesn’t exist, the observations that led astrophysicists to conclude it exists won’t go away. We would therefore need something else to explain the observations. The currently most plausible alternative explanation would be that we misunderstand the law of gravity. Personally, I would find this more interesting, because it could be a link to quantum gravity and possibly also explain dark energy.
That said, “dark matter” is so vaguely defined that it is impossible to falsify. In the best case, we can “implausify” it by coming up with a better explanation. At the moment though, modified gravity is equally good or bad as dark matter; each has its advantages and shortcomings. Personally, I find both explanations unsatisfactory. I would really like to see this problem attacked by AI.
Stacy McGaugh
Cosmologist, Case Western Reserve University; author of the blog Triton Station.
Astronomical observations have established beyond a reasonable doubt that there is a discrepancy between what we see and what we get in extragalactic systems—galaxies, clusters of galaxies, 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 come up short: the observed motions imply more mass than meets the eye: dark matter.
The inferred existence of dark matter is predicated 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 just that: an assumption. If dark matter does not actually exist, it implies that there is something more to learn about gravity. The observed discrepancies could be caused by a change in the force law rather than invisible mass.
One hypothesized change to the force law is a theory known as MOND [Modified Newtonian Dynamics]. MOND has made a remarkable number of predictions that were subsequently corroborated by observation. This should not happen in a universe made of dark matter, leaving us to wonder: is the invisible mass in the room with us now?
Juan I. Collar
Astrophysicist, Kavli Institute for Cosmological Physics at the University of Chicago.
Soon after Maxwell identified light as a traveling electromagnetic wave, the experimentalists set out to measure the presence of the medium through which it propagates, the luminiferous ether, or ether for short. The bias towards its existence was enormous, even if Maxwell’s equations provide a recipe for self-propagation of electric and magnetic fields that at no point relies on an ether. No evidence was found until the theory of relativity finally provided peace of mind almost two decades later. In hindsight there was no explanation other than pure, undistilled human bias, with a dash of lack of imagination, for our ether obsession.
Searches for particle dark matter have spanned at least twice as long as the ether craze, with a continuum of negative results to show for our efforts. The gravitational effects of what we have come to call “dark matter” are undisputed. To conclude that a new fundamental particle is behind those, or that said particle interacts through any other mechanism than gravitation, may well be retracing the obsessive-compulsive steps of the experimentalists post-Maxwell’s realization.
Short of a new Einstein putting us out of our misery, a solution (compassionate at this point) might come from observational astrophysics. Zero to no information on the nature of dark matter particles (constraints on mass, type, and coupling) has been provided from that field to the experimental community looking for those. An emphasis on providing this guidance could refocus the present willy-nilly disarray of searches for a multitude of particle possibilities into efforts with a higher chance of success.
Kyu-Hyun Chae
Physicist, Sejong University, South Korea.
The concept of “dark matter” has some similarity to the historical concepts of “epicycles” or “ether” in that they are logical consequences of well-established paradigms/ and theories. Eventually, epicycles and the ether were disproved and led to immense scientific revolutions. Dark matter is a logical consequence of standard gravity due to Newton and Einstein. The dark matter paradigm has been the default view for nearly a century since Fritz Zwicky and other astronomers noticed gravitational anomalies in astronomical systems such as galaxy clusters and galaxies in the 1930s and afterwards. Apparent successes of dark matter in explaining astronomical data may have some similarities to apparent successes of the epicycle-based Ptolemaic geocentric model in explaining observed planetary motions on the sky.
As epicycles did not eventually survive because of smoking-gun evidence such as Galileo’s observation of Venus’s phases, 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 come from observations that show direct discrepancies with standard gravity. Scientific papers presenting this evidence are already being published in recent years. In particular, detailed observations of internal dynamics of wide binary stars and galactic rotation curves will provide decisive evidence in the coming years. I think that a new scientific revolution is (likely to be) under way and will be cheered by many scientists. It is perhaps ironic for history to keep showing that scientific revolutions come at the expense of seemingly perfect physical theories. This is an exciting time in gravity and cosmology!
Don Lincoln
Senior scientist, Fermilab; science communicator.
The debate on whether dark matter or modified physics is the answer to cosmic mysteries like rapidly rotating galaxies is both interesting and spirited, making it a fascinating topic for physicists. For years, I leaned in the “modified physics” direction; however, two observations have put me firmly in the dark matter camp. The Bullet Cluster firmly favors the dark matter conjecture, as does the observation of ultra-diffuse galaxies in the NGC-1052 group. These galaxies, called DF2 and DF4, appear to be governed by the currently accepted laws of physics. In a moment of cosmic irony, the existence of galaxies that appear to contain no dark matter is strong evidence for the existence of dark matter. If modified physics were the answer to the question, there would be no galaxies that rotate as Newton’s laws predict.
Assuming that particulate dark matter is real, the question becomes “what is its nature?” The answer is elusive. Sensible dark matter candidates have been hypothesized with masses ranging from 10 to 11 times the mass of an electron up to the mass of a mid-sized asteroid. Experiments have ruled out classes of possible dark matter candidates with specific properties in the range of 1 to 1000 times the mass of a proton, although many possible candidates remain viable. Whether we ever directly detect dark matter particles depends crucially on how they interact. If it turns out that they only interact gravitationally, we may never detect them. If we ever hope to know the answer, the only path forward is to continue to look for dark matter in as many creative ways as possible. Hopefully, one lucky group of researchers will make the observation that answers this very fascinating question.
Indranil Banik
Astrophysicist, University of Portsmouth.
Naturally, not all astronomers are happy with dark matter. To make do with only the visible mass, we have to relate their visible matter to the observed gravity, inferred from how quickly galaxies rotate. I believed in MOND for a period of ten years, during which I worked on testing MOND in various ways. By conducting a detailed analysis of Gaia observations of thousands of wide binaries in the Solar neighbourhood, I was able to conclusively disprove the MOND prediction.
All this is even before considering evidence from cosmology that also suggests you need dark matter. I recently published a paper on galaxy formation without dark matter, which in the standard picture is required to act as a seed for the galaxy to form around. Whether you consider outer Solar System planets like Saturn, wide binary stars in the Solar neighbourhood, or the fact that galaxies formed quite rapidly after the Big Bang, the observations are far better explained by using standard gravity combined with dark matter. I have not come to this conclusion by staying within my comfort zone, since I did believe in MOND for a decade and published many papers and a long 150-page review in that time advocating it. However, I now believe that if there is something wrong with our theory of gravity, the inaccuracy is very small until you get to distances of at least 100 million light years. On even larger scales, there are reasons to think that our theories do not work well. However, modifications to gravity on 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, then very little sense can be made of the Universe even if one is prepared to modify gravity, since there is very little flexibility in how one could hope to do that in a universe with only visible matter. In short, I believe that dark matter does exist and is the main component of galaxies and larger structures like galaxy clusters.
Read the full article here
