9 of the Biggest Unanswered Questions About Dark Matter

The questions surrounding the dark matter are some of the most important physics mysteries of our modern age. The existence of dark matter was initially proposed by Dutch astronomer Jacobus Kapteyn in 1922, using research on stellar velocities. Then, in 1933, Fritz Zwicky noticed something odd in the distant Coma cluster of galaxies. 

The Swiss American astronomer discovered that the mass of all the stars in the Coma cluster of galaxies provided only about 1 percent of the mass needed to keep the galaxies from escaping the cluster's gravitational pull. In fact, dark matter was initially called "missing matter," because astronomers could not find it by observing the universe using any part of the electromagnetic spectrum. It was not until the 1970s that dark matter was officially confirmed to exist by American astronomers Vera Rubin and W. Kent Ford. 

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So, what is the current hype surrounding dark matter? You might have seen it referenced in science fiction films or TV shows or even read about it in the news, as the scientific community works to understand it better. Properly understanding this missing matter could help scientists to better understand the precise nature of the universe and perhaps on even how our universe will end. 

Dark matter is far more prevalent than we think, according to some theories. The amount of dark matter could show us if the universe is expanding, or if it might collapse sometime in the distant future, or stop moving altogether. Dark matter could also help researchers to better understand how gravity works, or how galaxies form. The list goes on. However, there are still many unanswered questions regarding the dark matter that need to be tackled before astronomers can move forward. Today that is what we are going to explore. Let's get started. 

1. What exactly is dark matter? 

If you are still not sure what dark matter is, you are not alone. Although many researchers believe that dark matter makes up 85% of the matter in the universe, there is not yet an agreement on what exactly dark matter is. Even more, the more we study dark matter, the murkier things seem to get. However, there are two main theories for the nature of dark matter.

One is the suggestion that dark matter is made up of ordinary but hard-to-see dead stars or huge cold planetary objects. These objects would accumulate inside galaxies in a "halo" and so are called "Massive Compact Halo Objects", or MACHOs.

The other popular theory is that dark matter is composed of undiscovered particles which were created in the Big Bang and exist everywhere. These are called Weakly Interacting Massive Particles or WIMPs. This is currently the leading theory.

Physicist Don Lincoln of the U.S. Department of Energy's Fermilab expanded on this idea in his article for Livescience stating, "We have never directly observed dark matter, but we know a great deal about what it must be: It must be massive (because it affects the rotation of galaxies); it must be electrically neutral (because we can't see it); it must be different from ordinary matter (because we see no evidence for it interacting with matter in the usual ways), and it must be stable (because it has existed since the dawn of the universe). These properties are unequivocal."

"However, we don't know exactly what it is. In the most popular generic theory, the dark-matter particle is called a WIMP, for weakly interacting massive particles. WIMPs are kind of like heavy neutrons (but definitely not neutrons), with a mass of 10 to 100 times heavier than a proton. They were created in great quantities during the Big Bang, and a small relic remainder persists to this day." 

Yet, even if these WIMPs are barely detectable, they should be everywhere, and we should be able to interact with them in some way. This was the prevailing theory for an extended period of time. Over the decades, researchers have built machines to detect the dark matter WIMP particles with no avail, until recently.

One dark matter detector in Italy recently came back with positive results. Yet, the DAMA experiment is very controversial, as other dark matter detectors around the world, offering conflicting results. At the moment, we do not yet have definite proof that these theoretical particles even exist.

2. Shouldn't dark matter interact with something? 

Yes, absolutely. But this is much harder than you might think. First, researchers can only study dark matter by seeing how it affects the universe around it. Up to this point, the only thing that has been observed is the gravitational effects of dark matter, which further clouds our ideas on the subject.

From previous studies of the dark matter phenomenon, we do know that it impacts celestial objects. But what if dark matter is not a particle? Perhaps it's a field? Or maybe we do not fully understand how gravity works. 

In 2015, researchers observed four large clumps of what they believed to be dark matter surrounding four colliding galaxies. Their observations showed one of the clumps weirdly lagging behind its galaxy. Not only did this appear to confirm that dark matter affects galaxies, but it also demonstrated that dark matter could interact with other dark matter. Yet, we have yet to see if dark matter interacts with ordinary matter. 

3. Is there a way to properly study dark matter? 

As mentioned above, researchers have tried to detect particles of dark matter for years, with no success. However, if the WIMP theory is correct, it would be very difficult to measure them properly. Yet, if we assume these particles are traveling through space then at some point, dark matter should interact with a more familiar form of ordinary matter, like a proton or electron.

To measure this, researchers have built experiment after experiment to study interactions of ordinary particles deep underground, where they are shielded from interfering radiation that could mimic a dark-matter-particle collision. However, even the most recent Chinese PandaX experiment has yet to produce conclusive results. One prevailing theory to explain this is that the particles of dark matter are actually much smaller than WIMPs and so very difficult to detect. 

4. Could dark matter be made of more than one particle? 

This question does make a lot of sense. After all, ordinary matter is not just made up of protons and electrons. It also contains a host of "exotic" particles such as neutrinos, muons, and pions. It would not be too far off to believe that dark matter has a similar "exotic" mix of particles. "Dark matter particles would essentially consist of heavy 'dark protons' and light 'dark electrons'" says Charles Q. Choi.

"They would interact with each other far more than other dark matter particles to form 'dark atoms' that use 'dark photons' to interact through a sort of 'dark electromagnetism,' much as regular protons and electrons interact through photons in conventional electromagnetism to build the atoms making up the stuff of everyday life. If dark atoms are possible, they could react with each other for dark chemistry, much as regular atoms interact chemically."

5. What about dark forces? 

No, we are not talking about the Darkside of the Force. Yet, it could work similarly for all we know. As mentioned above, dark matter could be comprised of what is called dark particles. The way that dark protons and dark electrons interact with each other could explain why dark matter clumps together, forming spherical halos around galaxies, stars, and planets.

This could also open the possibility of the existence of dark photons. What are dark photons, you ask? In short, they are photons exchanged between normal particles that give rise to the electromagnetic force, except they would be felt only by dark matter particles. Theories like these could open a Pandora's Box of a whole other side to the universe. 

6. What if dark matter is the matter of axioms? 

While some researchers continue to focus on the WIMP theory, searching for weak particles, attention has been shifted to another particle, axioms. These are ultra-lightweight particles, billions of times lighter than the electron. They are thought by some to be an excellent candidate for dark matter, for multiple reasons.

First, their invisible presence would explain why the universe is much heavier than it looks. Secondly, the particle would also show why the two fundamental forces that shape atomic nuclei follow different rulebooks. The Axion Dark Matter Experiment (ADMX) at the University of Washington is currently leading the charge for axiom and dark matter science. 

The axion is a candidate for dark matter, since, just like dark matter, it can't really interact with regular matter. This aloofness also makes the axion, if it exists, extremely difficult to detect. This strange particle could also help solve a long-standing conundrum in physics known as "the strong CP problem".

7. If dark matter exists everywhere, shouldn't it exist in every galaxy? 

One of the leading theories around dark matter is that it plays a vital role in the formation of galaxies. It is believed that dark matter plays a role in controlling and organizing the formation of large celestial structures. However, researchers have now found a galaxy that appears to have no dark matter at all.

If we think of dark matter as the scaffold that holds the universe together, why would a galaxy be missing these crucial structures? One answer is that dark matter might not play as big a role in the formation of celestial bodies as was previously thought. 

8. What about our positive detection of dark matter particles?   

As mentioned above, research from the Italian DAMA project, which reported the finding of dark matter, is highly controversial. To find the mysterious and elusive dark matter particles, researchers around the world have created underground detectors to try and observe WIMP particles interacting with ordinary matter.

To date, DAMA is the only project that has succeeded in demonstrating the existence of dark matter particles. Other leading detector projects located in different places around the world have reported conflicting results. To this day, the accuracy of the DAMA results is hotly debated.

9. Can ordinary particles decay into dark matter?

What if ordinary particles actually turn into dark matter particles? After all, we already see something similar occur in electrons and neutrons. A lone neutron will slowly decay into a proton while an electron will decay into a neutron. Some researchers believe that 1% of these particles actually decay into dark particles. Bartosz Fornal and Benjamín Grinstein from the University of California, San Diego, propose a solution to this discrepancy that assumes neutrons decay 1% of the time into dark matter particles. Because beam experiments would not detect these decays, their inferred neutron lifetime would be longer than the actual value. 

Do you think we will finally understand dark matter in the future?