What is Dark Matter?
- Fiona Hamilton
- 3 days ago
- 4 min read
When you hear the phrase “dark matter,” you may immediately think of some kind of hocus pocus dark magic. If you know it has something to do with space, you may think it has something to do with black holes, such as the one in the artist’s interpretation below.
While having similar names, dark matter and black holes aren’t directly related. Black holes are named so because the force of gravity from the black hole’s singularity is so strong that even light cannot escape. Dark matter, on the other hand, doesn’t even react with light. It is ‘dark’ because there is no way to see it.
Dark matter makes up about 80% of all of the universe’s matter, yet it is completely invisible to us! So, what is dark matter?
Essentially, we don’t really know! There are various ideas, though, about what dark matter could be, including WIMPs, axions, sterile neutrinos, or even primordial black holes.
I know I just said that dark matter and black holes aren’t directly related, so I’ll explain that last one first. The idea is that dark matter could be really, really tiny black holes from the beginning of the universe. However, this would be hard to confirm and is already a pretty long shot. So, still probably not directly related.
Sterile neutrinos aren’t proven to exist yet, but if they did, they are hypothesized to only interact with gravity and not other forces, which would track with what we know about dark matter. Sterile neutrinos would also travel in different directions from their spins than other types of neutrinos.
Axions are one of the top candidates for dark matter. Axions would be like a strange, invisible fluid spread throughout the universe. They are ultra-light particles, and the most appetizing prospect they bring to dark matter is how they could possibly solve other questions in the physics world.
WIMPs are Weakly Interacting Massive Particles, and are also one of the top candidates for dark matter. WIMPs would interact with gravity but not with light or electromagnetism. Before WIMPs became popular, MACHO (MAssive Compact Halo Object) particles were considered for dark matter. However, if they did contribute to dark matter, it wouldn’t be very much, and so research has turned to focus on possibly more important prospects.
Okay…So, how do we even know that dark matter exists if it doesn’t interact with light?
First off, gravity doesn’t work right at a large cosmic scale unless we add some mass. The stars that reside at the edges of galaxies move much faster than they should if we only considered visible matter. Most dark matter seems to reside in a ‘halo’ around the regular matter, from around a galaxy to around a whole cluster of galaxies.
Also, gravity affects light, which is why we can’t see inside a black hole. As light travels to Earth from distant galaxies and their clusters, it is warped due to the large masses. This is called
gravitational lensing, and because of this, we know that there must be more mass out there than we can see. There is both strong and weak gravitational lensing, with weak being more common but requiring much more data to notice its effects. A cluster known as the Bullet Cluster, which was made through a collision of two smaller clusters, provides great evidence for dark matter through gravitational lensing. The collision between the two former clusters caused a shock wave, which we can see going through the gas. (See photo: Pink represents visible matter, and blue represents dark matter.) Gravitational lensing lets us see that the majority of the mass is not gathered where the majority of the gas is.
The Cosmic Microwave Background (CMB) is leftover radiation from the Big Bang that causes temperature fluctuations that tell us how matter is spread throughout the universe.
The CMB fluctuations only make sense with dark matter in the equation. This has helped scientists determine that dark matter makes up 27% of the total mass and energy of the universe.
The formation of the universe could only have turned out as it is now with the extra dark matter mass. We see this through computer simulations of how various galaxies and other grand-scale objects could form.
Some have speculated that dark matter doesn’t exist, that our idea of gravity is wrong. This isn’t widely believed, though. However, the most prominent theory is known as the MOdified Newtonian Dynamics (MOND). Essentially, MOND modifies how gravity works at low accelerations, like at the edges of galaxies. However, this doesn’t explain the evidence granted through gravitational lensing and the CMB, so this theory isn’t widely accepted.
Dark matter makes up 27% of the universe’s contents and 80% of the matter. (Dark energy makes up 68% of the universe, but that’s something else entirely.) We don’t know much about it, but it has an important impact on our universe. From machines hidden deep underground and away from cosmic rays, to particle colliders trying to create dark matter, there are scientists from around the world who are studying dark matter. Who knows whether we’ll ever be able to figure out dark matter, but either way, we’ll discover new concepts and learn more about the world somewhere along the way.
Sources:
On dark matter:
More on sterile neutrinos:
Photos through https://www.esa.int/ESA_Multimedia/Images:
Photo 1- courtesy of ESA, NASA, JPL-Caltech
Photo 2- courtesy of ESA, X-ray: NASA/CXC/CfA/M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/U.Arizona/D.Clowe, Lensing map: ESO WFI
Photo 3- courtesy of ESA, Hubble & NASA, D. Thilker
Cover Photo (Planetary Nebula)- courtesy of NASA, ESA, STScI
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