The universe holds many mysteries, but none as elusive, or abundant, as dark matter.
Dark matter makes up around 85 percent of all matter in the universe and about 27 percent of its total mass, outweighing visible matter around six to one.
Researchers are quite confident dark matter exists — but they haven’t been able to prove it exists with direct, observational evidence. That doesn’t mean we’re asked to believe in it based on faith: Through a process of elimination, the science is real and tangible. It’s also incredibly complicated to find because of its elusive properties.
Helping you find your way through this elusive mass, Inverse breaks down the meaning of dark matter, how it differs from dark energy, and why it is so hard to detect.
What is dark matter?
In 1933, astronomer Fritz Zwicky noticed something missing from the universe.
Zwicky was observing galaxy clusters when he noted that the total mass of matter was greater than what was being directly observed. He calculated the difference in visible versus invisible matter based on the motions of the galaxies near the edge of the cluster.
Zwicky then noticed that there was a difference between the matter he had calculated based on the number of galaxies and their brightness — and the matter that he could directly observe.
Later on in the 1970s, astronomer Vera Rubin followed up on Zwicky’s work by estimating that most galaxies must contain about six times as much dark matter as visible mass.
Priyamvada Natarajan, an astronomy professor at Yale University, explains that the two pieces of evidence for dark matter come from the motion of stars and galaxies, and the bending of light in the universe.
“Both of these reflect gravitational force since gravitational force dictates the motions of stars and galaxies,” Natarajan tells Inverse. “Therefore, the matter that you see should be commensurate with the motions detected because they provide the gravity.”
“If all you see is all that provides the gravity then it’s consistent with the motion, and that is not the case,” she adds.
Therefore, some form of mass has to be in between cosmic bodies holding them in place through sheer gravitational force. That missing mass was dubbed dunkle materie by Zwicky, or dark matter.
Although they strongly believe it exists, scientists aren’t exactly sure what makes up dark matter. The two leading hypotheses are summed up by two stellar acronyms:
- WIMPS and
WIMPS — One hypothesis is that it may be made up of exotic particles such as WIMPs (Weakly Interacting Massive Particles). WIMPs are hypothetical particles that don’t interact strongly with other particles, nor do they absorb or emit light.
In some models of dark matter, on the rare occasion that two dark matter particles interact, they end up destroying each other and emit powerful gamma rays.
Scientists aren’t sure which particles could qualify as WIMPs, but certain kinds of neutrinos could be a strong candidate. Neutrinos are neutral, subatomic particles with tiny masses close to zero — smaller than the mass of any other known particle.
Meanwhile, sterile neutrinos are a hypothetical particle that only interacts through gravity and not through any standard model of particle interactions, making it an excellent candidate for dark matter.
Another dark matter candidate in the WIMPs hypothesis is axions, particles that may have been produced in the early universe. Similar to the way dark matter acts, axions are believed to be slow moving particles that could clump together and potentially influence the motion of galaxies.
MACHO — Another possible explanation for dark matter is the MACHO (Massive Astrophysical Compact Halo Object) hypothesis. MACHOs are believed to be made up of ordinary matter that emits little to no light and drifts through space as wandering cosmic bodies.
MACHOs could be neutron stars, brown dwarfs (a kind of failed star), black holes, and/or rogue planets. These could serve as good examples of dark matter since they emit little to no light.
What is dark energy?
Along with dark matter, there’s a separate entity — making up even more of the mass of the universe — called dark energy. Although shrouded in mystery, dark energy makes up around 68 percent of the mass of the universe.
Despite its abundance, dark energy has never been directly observed nor measured. One of the telltale signs that dark energy exists is its effect on the expansion rate of the universe.
In the 1990s, astronomers discovered that not only was the universe expanding but that it was expanding at an accelerated rate as opposed to the expected constant rate of expansion. There had to be a factor that was countering the effect of gravity, causing the universe to expand at a faster speed.
That factor became known as “dark energy.”
Scientists are still not sure of the properties of dark energy, but one leading theory suggests that dark energy is a property of space.
According to Albert Einstein, space is not just emptiness. Instead, space could have its own form of energy.
If space has its own energy, the more space there is, the more of that “dark” energy exists. The more of that energy there is, then the faster the universe will expand. That’s why the universe is expanding at an accelerating rate.
Dark matter vs. dark energy
Although they both may seem related, dark matter and dark energy may actually be competing against one another in the universe: one that pulls objects together as the other tries to tear them further apart.
As the more abundant of the two, dark energy exerts its influence on the entire universe while the effects of dark matter can be observed on individual galaxies, as well as the universe at large.
Dark matter holds cosmic objects together, acting as a force of attraction. On the other hand, dark energy is an anti-gravity force, repelling objects from one another in a way that would cause the universe to expand.
Scientists aren’t sure if dark energy and dark matter are somehow related to one another or simply two stubborn and stubbornly invisible forces of the universe.
Why are dark matter and dark energy so hard to detect?
As the name suggests, dark matter and dark energy are, well, dark.
These two forces emit little to no light, so scientists have not been able to spot them with the telescopes that we have developed so far.
In the meantime, scientists are only able to suggest the existence of dark matter based on the effect that it has on the matter that they can see: observable matter.
Although dark matter cannot bend light itself, mass, including invisible mass, bends spacetime. Light follows the curve of spacetime, so the effect of dark matter can be seen through the lensing effect created by this curvature.
Does everyone believe in dark energy?
Scientists are quite confident that dark matter and dark energy exist. However, they just haven’t been able to prove it with direct, observational evidence yet.
Due to their enigmatic properties, dark energy and dark matter can often seem unreal which is why they are used in fiction like comic books, video games, and movies.
For example, in the CW television series The Flash, Barry Allen gets his super-speedy powers from dark matter. Meanwhile, Leela’s pet Nibbler in the series Futurama defecates dark matter, which is then used as fuel for their spacecraft.
Is there evidence against dark matter?
As scientists go on the hunt for dark matter and come up empty, claims that it does not exist begin to resurface.
A recent study that came out in January 2021 favored a dark matter alternative, Modified Newtonian Dynamics. This suggests that instead of dark matter acting as a supporting force to gravity, that gravity itself doesn’t just depend on the mass of an object, but also on the gravitational pull of other massive objects in the universe.
However, most astronomers support the dark matter theory instead.
How can we prove the existence of dark matter?
Scientists have come up with all sorts of ways to detect dark matter if it’s a WIMP.
The Large Hadron Collider is the world's largest and highest-energy particle collider. The collider is made up of a 27-kilometer ring of superconducting magnets and two high-energy particle beams that travel at close to the speed of light before they are made to collide.
The collider could theoretically produce dark matter particles, which would escape through the detectors unnoticed. However, they would carry away energy and momentum, and scientists would be able to infer their existence from the amount of energy and momentum that would be missing in the aftermath of a collision.
The XENON experiment in Italy is a sensor-lined tank filled with 3,500 kilograms of liquid xenon, a type of gas used in light manufacturing, that hunts for the hypothetical particles that could make up dark matter.
Inside the pool that’s buried thousands of feet beneath the Gran Sasso mountain, scientists look for the ripples that may be created by unknown particles.
Meanwhile, the Sanford Underground Research Facility in South Dakota is running its own Xenon dark matter experiment with a chamber containing seven active tonnes of liquid xenon to search for dark matter particles.
Japan is also anticipated to build the world’s largest neutrino detector that will contain 260,000 tonnes of ultrapure water to capture neutrinos as they collide with water atoms.
Even with these large-scale experiments, dark matter still lurks in the shadows, undetected yet as elusive as ever.