What even is dark matter?

What Is Matter?

Anything that has a mass and takes up space is a form of matter — and it’s all around us.

Every form of matter that we can interact with –- the rain that falls from the sky, the bibimbap bowls from the Tutor Campus Center and even the air that we breathe  —  only comprise around 15% of all the mass in the universe. Dark matter accounts for that aforementioned remaining 85% but is a type of matter that doesn’t interact with any form of light from the electromagnetic spectrum (thus dark).

According to the ⴷCDM model of cosmology, dark matter is the foundation that the universe is built on.  However, since dark matter can’t absorb, emit or reflect light, it is imperceptible across all wavelengths, making direct observation impossible. Nearly a century after its initial proposal, scientists are only now beginning to theorize on its identity. 

Thankfully, dark matter does interact with gravity. So even though “invisible,” dark matter has mass, allowing for careful experimentation and analysis that can determine what specific particles are present and thus responsible for the majority of the universe’s matter and therefore mass.

Investigating dark matter is critical because it was the driving force for the creation of stars and galaxies as we know them now, and thus knowing more about dark matter will give us much more depth into our understanding of the universe. We may know the ordinary 15% of matter pretty well, and now we can strive to understand the other 85%.

How They Exist in the Universe

In the last few decades, physicists have proposed dozens of possible candidates for particles that may be identified as dark matter by using the information gathered from natural phenomena and computer simulations. There are three main categories of dark matter conveniently named: hot, cold, and warm dark matter. 

One can think of the “temperature” of the categories as a metric of particle speed. Much like the root-mean-squared velocity of gas particles in chemistry, the hotter the temperature, the faster the movement will be. The different speeds of these three types of matter mean they serve different purposes in the universe! 

It’s theorized that cold dark matter moves slowly, building gravitational “wells” that would have otherwise trapped ordinary matter together, clumping the “wells” into the large structures we can observe in the universe today. Oppositely, hot dark matter would have moved past those wells, with its inherent speed, not contributing much to the building of such large structures.

Albeit that hot dark matter is assumed to comprise only a small percentage of dark matter, it is still important. 

Categorizing Candidates

In many cases, already experimented-on particles later are recognized for their congruence to the expected behavior of dark matter and identified as candidates. 

Relic neutrinos are virtually massless subatomic particles that carry no charge. Intuitively, as their name suggests, relic neurotrines are relics of the ancient universe that were released into the universe during the moments immediately following the Big Bang. Broadly, neutrinos are thought of as existing in three forms: the electron, muon, and tau neutrinos. Recent experimentation and data from certain reactors point to a fourth state, the sterile neutrino. Groups such as the KATRIN group wish to prove the existence of the sterile neutrino with experimentation. They utilize precise analysis of certain atoms to experiment on their existence, monitoring the energy spectrum of electrons in the hopes of noticing distortions by sterile neutrinos.

Axions were originally proposed as an arbitrary “very light” particle — maybe even lighter than neutrinos, already multiple orders of magnitude lighter than an electron — by Frank Wilczek in 1978 to organize mismatching calculations about the strong force. The strong force analyzes force within subatomic particles such as protons or neutrons. Both protons and neutrons can be split further into quarks, where three quarks combine to create one proton or neutron. Seeing as it is still a theoretical particle, scientists have come up with a variety of experiments to detect axions. One brewing method is being researched by the HAYSTAC group. They plan to utilize a potent magnetic field that stimulates axions converting into photons. If the content is analyzed and scientists determine it to be truly axions, their existence will be proven. 

As experiments began on these particles, the relic and sterile neutrino, as well as the axion, they were recognized to be possible candidates for dark matter. 

As the traces of relic neutrinos were being discovered by backtracking the history of the universe, scientists realized that the fast-moving behavior of relic neutrinos, accompanied by the early expansion of the universe, made it a perfect candidate for hot dark matter.  Silvia Pascoli, a theoretical physicist at the University of Bologna, had an interview with Symmetry Magazine where she attested to the importance of relic neutrinos. “Relic neutrinos are currently the only known component of dark matter,” she said. Their notoriety in the scientific community is indisputable; they are the only dark-matter-behaving particles we can confirm experimentally. 

The sterile neutrinos, unlike that of relic neutrinos, were calculated to have a slower speed. They would be able to stream freely through the universe, but not fast enough to brush over the large structures of the universe. Warm dark matter is the theoretical middle ground between hot and cold dark matter. Although not fast enough as hot dark matter to brush over large structures, or slow enough like cold dark matter to “clump” large structure development, warm dark matter will provide the nuanced amount of speed to create the universe as it is. 

Sterile neutrinos offered a theoretically perfect fit to the properties of warm dark matter, being light and moving at intermediate speeds. With a proper detection, they will be able to rephrase the ⴷCDM model of the universe and offer much more insight into the identity of dark matter. 

Axions, with their infinitesimal mass and lack of charge, were analyzed to be a perfect candidate for cold dark matter. This was thanks to the slow speeds that axions would theoretically exhibit, thus being a form of cold dark matter. Cold dark matter is often regarded as comprising the largest amount of dark matter. It is essential to the current theory of cosmology, as its abbreviation, CDM, is in the current standard model of cosmology, the ⴷCDM model, viewed as the skeletal frame on which the universe is being built and expanded. Thus, the discovery of the axion will give us a better understanding of the big picture of the universe.

The search for dark matter is not a simple attempt to discover a new particle, but one striving to form the baseline of human knowledge on the architecture of the universe. Whether that be the remnants of the Big Bang evident in hot dark matter, or the cold axions that built the galaxies we see in space, or the Goldilocks of two, warm dark matter, knowing more about dark matter advances physics in the realm of cosmology and astrophysics. Understanding the identity of dark matter will not only add to the standard model of cosmology but also allow us to conduct more research on topics such as dark energy, until the day scientists can proudly say that we understand the universe in its entirety. As experiments such as HAYSTAC or KATRIN work to inch us closer to this goal, we are ready to make the next great leap in physics, finally shining light on the dark parts of the universe.