What Are Dark Matter and Dark Energy, Anyway?
Sophie Bilanin '26
Either spoken as a comfort or a warning, we are often told that our existence as highly cognitive beings on a life-sustaining planet is not particularly special. We are simply one planet out of eight in the solar system, which is only one out of 3,916 in the galaxy, which is only one out of possibly two trillion in the universe, and so on to infinity. However, the baryonic matter, consisting of electrons, protons, and neutrons, that comprises these physical objects in the observable realm makes up only a small percentage of the actual universe. According to NASA, “It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe” (NASA, 2023).
So what are these mysterious ulterior forces known as dark matter? In 1998, the Hubble Space Telescope’s (HST) observations of distant supernovae revealed that the universe expanded more slowly in the past than today. This directly contradicted the commonly accepted belief during the early 1990s that gravity slowed the expansion of the universe. One scientist compared this revolutionary finding to “throwing a set of keys up in the air, expecting them to fall back down, only to see them fly straight up toward the ceiling” (National Geographic, 2017). Theorists turned to Albert Einstein’s discarded theory of gravity for an explanation. This theory contained a “cosmological constant” represented by the Greek letter lambda (λ). The cosmological constant was first introduced by Einstein as a “deus ex machina” solution to the problems in the theory of general relativity, which predicted either the eventual expansion or contraction of the universe. In order to maintain his belief in a static universe, Einstein added this constant as an opposing force to gravity. Later, Russian mathematician Alexander Friedmann proposed the model now known as the “Big Bang Theory,” and an HST study proved the expansion of the universe, leading Einstein to discard his cosmological constant and dub λ his “greatest mistake” (NASA, 2012).
After the HST’s more recent discovery of the accelerating expansion of the universe, the cosmological constant was revived to explain this baffling phenomenon known today as dark energy. Unlike what we don’t know about dark energy, what we do know is very limited: it is the universe’s dominant form of energy, rapidly increasing its expansion against the force of gravity. One theory by Einstein suggests dark energy to be a property of empty space. Einstein realized the possibility for more space to come into existence and predicted empty space to possess its own form of energy that would not weaken as space expanded, but rather grow, causing the universe to expand even faster. Another theory focuses on the formation and disappearance of virtual particles to give energy to empty space. However, the actual energy produced by these theoretical particles was calculated to be 10120times larger than the estimated value of λ observed by astronomers, a far cry from solving the problem of dark energy. Other explanations include a type of energy-fluid that fills all space and counteracts the effects of gravity referred to as quintessence, or simply that Einstein’s previous theory of gravity is incorrect, calling for a complete reassessment of all known galactic behaviors. No current research has been able to provide evidence for any of these theories, leaving dark energy a mystery.
Prior to the HST’s 1998 discovery, cosmologists determined a possible end of the universe known as “the Big Crunch.” In this scenario, the universe would ultimately stop expanding and collapse in on itself due to the overwhelming force of gravity, creating a black hole singularity. Some versions of this theory also involved a potential reformation of the universe through another Big Bang. After this was disproved by revelation that the universe will not be overwhelmed by the force of gravity, but rather its counterpart dark energy, cosmologists devised “the Big Rip.” This theory suggests that dark energy will eventually overtake every one of the universe’s fundamental forces, ripping apart all of the bonds they formed. This will range from the destruction of galaxies to planets to the subatomic particles comprising atoms until nothing exists in the universe.
In 1937, Swiss astronomer and Caltech professor Fritz Zwicky was studying the movement of individual galaxies within the Coma cluster. This ensemble of about a thousand galaxies was found to orbit the cluster’s center at an impossibly high velocity for the previously assumed mass of the cluster. Despite ranking among the largest galaxy clusters in the observable universe, Coma did not contain nearly enough galaxies to account for these extreme speeds. These galaxies surpassed the speed of their escape velocity, or the speed necessary for any galaxy to leave its orbit around the cluster, causing Coma to fly apart completely. No possible explanation remained for the continued survival of the Coma cluster, save for the introduction of a mysterious new substance called dark matter.
A study surrounding the rotation of individual galaxies by U.S. astronomer Vera Rubin proved the formerly dubious existence of dark matter in 1970, exacerbating the problem of the missing mass across the universe. The average discrepancy between mass from visible objects and the mass estimated by gravity calculations across the universe reveals that dark matter has six times the gravity of visible matter. Further proof as to the strange nature of dark matter can be found in its “[un]willingness to participate in the atomic and nuclear forces that shape matter as we know it,” (Tyson, 2017). One helium nucleus exists for every ten hydrogen nuclei due to nuclear fusion during the first few minutes after the Big Bang. However, if dark energy had been involved in nuclear fusion, calculations show that there would be more helium relative to hydrogen in the universe. This eliminates the possibility of dark matter existing as some other form of ordinary or visible matter, making it an entirely separate phenomenon. Dark matter exerts gravity in the manner familiar to modern science while remaining completely undetectable and without interacting with ordinary matter.
The European Organization for Nuclear Research (CERN) is home to the Large Hadron Collider (LHC), the largest and most powerful particle accelerator on the planet. Theories speculate that dark matter particles are light enough to be produced by the LHC, so the machine’s detectors are used to look for the main signature of the presence of a dark matter particle through missing transverse momentum in collisions. Because momentum will remain constant in a system unless it is acted upon by an outside force, the experiment aims to find a nonzero total momentum post-collision to determine if momentum has been carried away by undetected dark matter particles. However, without any recent experimental breakthroughs with the LHC, the question still remains as to dark matter’s composition. Some scientists regard it not as another form of matter, but as a flaw in our current understanding of gravity, or as another form of gravity beyond the work of Newton and Einstein.
The May 2027 launch of NASA’s Nancy Grace Roman Space Telescope promises advancement in the field of dark matter research. Roman’s images will be used to study tidal streams pulled from globular clusters, i.e., a group of stars orbiting our neighboring Andromeda galaxy. They aim to “pinpoint a greater number of examples of these tidal streams, examine gaps between the stars, and ideally determine concrete properties of dark matter,” (Blome, 2024). Additionally, CERN’s plans for the Future Circular Collider, “a huge new particle accelerator that would dwarf the Large Hadron Collider,” would allow for increased efforts in dark matter and dark energy research with the finalization of its electron-positron collider in 2048 (Celerier, 2024). Although dark matter and dark energy remain mysteries for the present, future projects
almost guarantee breakthroughs, and maybe the secrets of the universe will become known after all.
References
Breaking New Ground in the Search for Dark Matter. (2024, January 25). CERN. https://home.cern/news/series/lhc-physics-ten/breaking-new-ground-search-dark-matter Building Blocks—NASA Science. (2023). https://science.nasa.gov/universe/overview/building-blocks/
Celerier, P. (2024, February 5). Plan for Europe’s huge new particle collider takes shape. https://phys.org/news/2024-02-europe-huge-particle-collider.html
Dark matter and dark energy’s role in the universe. (2017, January 10). Science. https://www.nationalgeographic.com/science/article/dark-matter
Lea, R. (2022, November 24). What is Dark Energy? Space.Com. https://www.space.com/dark-energy-what-is-it
NASA’s Roman to Search for Signs of Dark Matter Slumps—NASA. (2024, January 17).
https://www.nasa.gov/missions/roman-space-telescope/nasas-roman-to-search-for-signs-o f-dark-matter-clumps/
Tyson, N. DeGrasse. (2017). Astrophysics for People in a Hurry (First edition). W.W. Norton & Company.
WMAP- Cosmological Constant or Dark Energy. (2012). https://map.gsfc.nasa.gov/universe/uni_accel.html