Dark Matter: The Universe We Can’t See

Empty space is not empty. In fact, it is full of dark matter, invisible particles that can’t be detected by any scientific instruments we have. The only reason scientists know that dark matter exists at all is because objects we can see are reacting to some invisible gravitational influence. 

Something is causing galaxy clusters to orbit faster than they should be able to. Something is causing light to bend as it passes through seemingly empty space in galaxy clusters. Dark matter is the theoretical cause for these strange observations. 

What is dark matter, though? Here’s a look at what scientists know about dark matter so far.

The Evidence for Dark Matter

Imagine a boat floating along in a seemingly empty tank. Anyone observing the boat would be perplexed – it is clearly floating on something, even rocking with a wave-like motion. However, the substance the boat is floating on isn’t visible. A human hand can even pass right through it. The boat should not be floating based on the usual laws of physics, and yet, it is. 

This is the experience of physicists studying dark matter. No scientific instruments have yet detected confirmed dark matter particles. However, there is evidence that indicates it is real. 

The Rubin and Zwicky Observations

In the late 1970s, astronomer Vera Rubin observed some unusual behavior from stars at the edges of their galaxies. Rubin expected to see stars further from the center of the galaxy moving slower, which would be in line with our current understanding of physics. However, Rubin found that objects in the farthest reaches of these galaxies were still moving extremely fast, much faster than they should have been. 

The only explanation for Rubin’s observations was the presence of some other mass that was exerting gravitational force at the outer edges of galaxies. This was the first intentional examination of dark matter, but other astrophysicists had also seen what Rubin was looking for. In the 1930s, dark matter was somewhat accidentally discovered by astronomer Fritz Zwicky, who is considered the “father of dark matter” today. 

Zwicky was studying data from Hubble’s observations of the Coma Cluster, a cluster of over 1,000 individual galaxies located about 323 million light years from Earth. Zwicky noticed that the individual galaxies in the Coma Cluster were moving so fast that they should theoretically have broken up the cluster. According to traditional physics, the Coma Cluster should not exist. This is where Zwicky stumbled on dark matter. He realized that in order for the Coma Cluster to exist as it does, something unobserved must be exerting other forces on the galaxies in the cluster. 

Rubin continued Zwicky’s thread with her research, where she got the first direct evidence of dark matter in the form of her galaxy velocity recordings. Ever since then, more and more scientists have joined in the search for dark matter, confirming Zwicky and Rubin’s findings. We can observe the effect of dark matter in the form of the behavior of visible objects in our universe.  

Gravitational Lensing

Scientists can also observe dark matter by observing how light bends as it moves through space. Galaxy clusters like those observed by Rubin and Zwicky contain a lot of invisible matter in the “empty” space between the galaxies. This massive amount of matter should have a corresponding gravitational pull, based on Einstein’s theories of general and special relativity. 

Scientists can use a trick known as gravitational lensing to observe dark matter’s gravitational force. As light passes by massive objects, their gravity bends the light. Astronomers choose a distant galaxy that lines up with a galaxy cluster. As the light from the distant galaxy passes through the galaxy cluster on its way to Earth, the gravity of the cluster’s dark matter bends the light. 

Dark matter may be invisible, but scientists can still observe how it affects things we can see. Gravitational lensing allows astrophysicists to study dark matter by studying how it interacts with normal matter. 

What Could Dark Matter Be Made Of?

Scientists have a few leading theories about what dark matter is, although none have been confirmed yet. The problem is, it is notoriously difficult to detect. In fact, there are dozens of dark matter research projects all over the world. However, only one ever detected anything, the DAMA/LIBRA project, and even that was subsequently ruled out by independent researchers. 

What We Know

Scientists know what they’re looking for based on how dark matter impacts the universe as astronomers have observed. Dark matter needs to be extremely massive. NASA scientists currently predict that dark energy accounts for 68% of mass and energy in the universe. Dark matter makes up an additional 27%. Everything else, most of the entire known universe, makes up only 5% of the universe’s mass and energy. 

Dark matter particles also need to ignore electromagnetic force. Dark matter isn’t “dark” in the sense that we think of darkness. It doesn’t absorb light. It doesn’t interact with light or the electromagnetic spectrum at all. That’s why it is so difficult to figure out what it is – all the tools we have for examining the universe are made for things on the electromagnetic spectrum. 

So, dark matter particles need to be extremely massive without interacting with anything on the electromagnetic spectrum, any matter or objects, beyond its gravitational influence. These invisible particles should be able to pass right through a person without them even noticing it. 

Beyond the Luminous Disk

One important aspect of Vera Rubin’s observations is where she observed the strange behavior within galaxies. Robin observed that stars farther from the center of their galaxies seem to be moving just as fast as those closer to the center. This behavior is occurring specifically beyond the luminous disk of galaxies. 

The luminous disk is the area of a galaxy where the most visible light exists. Beyond the luminous disk, objects orbiting in the galaxy should be slowing down. These objects are further from the massive object at the center of the galaxy, like a quasar or black hole, which creates the galaxy’s gravitational pull. The further away an object is, the weaker this object’s gravitational influence should be, causing objects to orbit with lower velocity. 

The fact that stars beyond the luminous disk seem to retain high velocities suggests that maybe where visible matter drops off, dark matter begins. That is, galaxies could be surrounded by a “halo” of dark matter located past the luminous disk.

WIMPs and Neutrinos

There are dozens of theories out there about what dark matter is made of. Two of the most well-known and hotly debated are WIMPs and neutrinos. WIMPs are weakly interacting massive particles, a general term for a group of mystery subatomic particles. These are hypothetical particles that projects like DARMA/LIBRA, mentioned above, were intended to find. No such projects have detected any WIMPs, though, even after years in operation. 

So, WIMPs are losing ground as a candidate for dark matter. What about Neutrinos? These subatomic particles make up three of the 17 “elementary particles”, the simplest known particles in the universe that make up everything else, including atoms. There are three types of neutrinos: electron, muon, and tau neutrinos. 

These particles fulfill some of the key characteristics for dark matter, such as their nearly non-existent interactions with the rest of the universe. Unfortunately, neutrinos are not nearly massive enough to account for the large mass that dark matter appears to take up based on astrophysicists’ observations so far. 
However, there may be a fourth kind of neutrino that could be a candidate. Physicists hypothesize the existence of “sterile neutrinos”, which only interact with other particles through gravity. This lines up with observations so far. Sterile neutrinos remain purely theoretical, though, and still have the issue of light mass that rules out the other types of neutrinos.

Axion Particles

Many scientists today are moving away from WIMPs and neutrinos and investigating another candidate: axion particles. These particles were first theorized while physicists were trying to solve another mystery, the “strong CP problem”, but they could also be dark matter. 

Axion particles are still theoretical – scientists have not proven their existence yet. However, simulations and mathematical models have shown that axion particles could potentially have enough mass to account for dark matter. Physicists simply need to figure out how to create an experiment that allows them to observe axion particles. 

The leading theory today is that dark matter is most likely some new kind of particle we haven’t encountered yet. Axion particles are among the leading theoretical particles that could potentially be that new particle. Dark matter could also be a combination of a number of different invisible particles, though. 

The Search for Dark Matter

Dark matter is one of the most perplexing mysteries of modern physics. It is a reminder that humans still have a lot to learn about our universe. We may be able to create VR video games and launch rockets to Mars, but we still don’t know what 95% of the universe is made of. 

Like Einstein and Newton, one day an astronomer will make history and change our perspective on the universe by discovering what dark matter is. Until then, scientists all over the world are exploring exciting theories and pushing the boundaries of science with cutting edge experiments. One thing is for certain: dark matter is out there and one day we will unveil its secrets.