Dark Matter and Dark Energy— 95% of our Universe

By Akhila Hiremath
A 2-Part Discussion of Dark Matter and Dark Energy
Published on December 2, 2023

Note: Best to view on computer because of formatting with the images :)

Introduction

I don’t know about you, but whenever I come across the topic of astronomy, I always have a slight existential crisis over how vast our observable universe is. It doesn’t help that my family loves to go to planetariums to periodically remind me of our insignificance in the grand scheme of things. Everytime I go to these shows, I always admire how astronomers undertake the grand task of bringing sense and order to the contrasting mystery of space. A few months ago, I happened to watch one of these shows that focused on the broad forces of dark matter and dark energy. Because these were complex topics being presented to the general public, the show simplified their descriptions: Dark matter is the driving, pulling force that holds the universe together, while dark energy is the driving, pushing force of expansion that pushes the universe further apart.

The vague description of these two gigantic forces being at play caught my attention, and I immediately wanted to explore the topics further. So, any free moment I got in between my painful schedule as a junior in high school, I spent researching dark matter and dark energy. Long story short: it was A LOT of information to digest, which you can probably tell when you look at the monstrosity that is my citations. I would read an article to answer one question, and leave with 10 more questions. Thankfully, I’ve worked my magic to compile most of what is known about the mysterious dark forces, including numerous hypotheses to cover the wide area of uncertainty regarding this topic. I'm specifically going to explore what exactly these forces are, their functions on the grand scale of the universe, possible theories of what they are made of, and how the universe is expected to be impacted by them in the future. Let’s begin with dark matter, the more well-known of the two.

Dark Matter

What is it and what does it do?

Dark matter makes up approximately 27% of our observable universe. In the introduction, I characterized dark matter as an invisible, “pulling force” of the universe, which makes dark matter sound almost like gravity, doesn’t it? Well, the truth is that dark matter and gravity work hand in hand, and that the strength of gravity determines the size and distribution of dark matter webs. Think of dark matter not as randomly distributed, but in a 3-dimensional, web-like shape that spans across our universe due to gravity. Thus, the dispersion of dark matter across our universe is appropriately named the cosmic web (the cosmic web also contains regular matter, galaxy clusters, etc.) Dark matter is only known to interact with other particles through a strong gravitational pull, much greater than the gravity that is exerted by observable matter. Our galaxy, although it may seem to hold immense amounts of matter that has its own gravity, wouldn’t have the gravitational strength to hold itself together if it weren’t for the existence of dark matter (giving weightage to its nickname as a “pulling force.”) Dark matter also accounts for the specific movements of observable matter, such as the rotations of galaxies, and the motions of star clusters, which are groups of stars held together by gravity.

An Artist’s Impression of the Cosmic Web

You might be thinking, “how did we even determine the existence of dark matter given that it’s an invisible force?” Since dark matter is composed of particles that do not absorb, reflect, or emit light, it would be difficult to study if it were not for its observable gravitational effect. The way that scientists even detected its presence is through a technique called gravitational lensing. What’s interesting about gravity from both observable and dark matter, is that it actually bends light, creating a distorted image of observable space from earth’s perspective. Scientists study dark matter distribution by observing the distorted image of our galaxy. By looking at how galaxies align with one another, they assume the spatial distribution (arrangement) of dark matter at certain places in the universe. They further used gravitational lensing to map out a more accurate view of our universe without the distortion of dark matter.

A 3D model of dark matter distribution in the cosmos

What could Dark Matter be made of?

There are several different hypotheses of what dark matter could be, but they are vague and prone to error because of our inability to observe dark matter directly. Knowing what dark matter is made of is essential in discovering more about its properties, and the mechanics of those properties as well. Luckily, there are several promising theories, with experiments running to gather evidence behind them. Most likely, dark matter is composed of multiple, complex particles that we have not discovered yet. The candidates must have the characteristics of what we know about dark matter currently: it must not interact with normal matter or light, but still possess a gravitational pull.

One of the earliest theories for the composition of dark matter is the WIMP hypothesis– quite the name. WIMP stands for “Weakly Interacting Massive Particles,” which is pretty self-explanatory. Scientists theorized the existence of these particles, stating that it could be a new type of neutral, massive particle that only interacts with other particles through gravity and a weak nuclear force. To fully define what a weak nuclear force is would be dragging you through particle and nuclear physics, so I’ll oversimplify it. Think of it as a small, directional force that’s limited to only affecting particles on the subatomic level. While the description of a singular particle is somewhat underwhelming, this would make some more sense on a larger scale, and the mechanics of this particle would likely imitate the behavior of dark matter.

So far, nothing has proved that WIMP particles actually exist. Because they interact so weakly, they would be difficult to detect in the first place. Searches for WIMP particles include using sodium iodide detectors that emit radiation upon possible WIMP collisions, or watching for slight temperature increases as an indication of WIMP collisions. Theoretically, these particles can’t be too large either, otherwise it would be inconsistent with the observed density of matter in the universe. However, this hypothesis is especially promising because of the simplicity of the WIMP particle, which has no electric charge, is stable, and is allowed to have a wide range of masses.

The other two hypotheses, which are relatively newer, are the axion and neutrino hypotheses. The axion is a theoretical particle ten trillionth the mass of an electron that would convert into a photon (observable light) in the presence of a strong magnetic field. Personally, I believe the axion hypothesis is the best candidate for the composition of dark matter. Since these particles are so small, it would explain the elusive nature of dark matter, and how difficult it is to study. Additionally, these particles naturally have little to no interaction with any sort of ordinary matter and are electrically neutral. Although axion existence hasn’t been proven yet, the XENON1T experiment has been utilized to detect them. The XENON1T experiment attempts to use liquid xenon to detect any dark matter particles, the results of which could hint to what dark matter is composed of. As of now, the data collected is inconsistent with the axion hypotheses. To be consistent with the data, there would need to be so many axions that it would heavily alter the evolution of stars heavier than the sun, which is not what we observe of the universe. However, these results do not definitively disprove the axion theory, as the discrepancies could be attributed to interfering forces like dark energy rather than dark matter itself.

The neutrino hypothesis is a little underwhelming compared to the other hypotheses that were discussed, but it is still worth mentioning. Neutrinos are produced when the nucleus, or the center of an atom, comes together or breaks apart. Like the other particles, neutrinos have no charge, rarely interact with matter, and don’t emit light. However, this particle has been proven to exist and is the most abundant in the universe because of their miniscule size. To put this into perspective, approximately 100 trillion neutrinos pass through the human body each second. Given their abundance and lack of interaction with normal matter, it’s logical to assume that neutrinos exist for a gravitational effect on the universe, which sounds similar to dark matter. But, at the end of the day, they are limited by their lack of mass and size, and could realistically only make up a small portion of dark matter. More theories suggest that maybe neutrino variations with a greater mass could make up dark matter, so it still remains a potential candidate.

Dark Energy

What is it and what does it do?

Even more prevalent than dark matter, accounting for an impressive 68% of the universe, is dark energy. Dark energy is a “pushing force” responsible for the accelerating, outward expansion of our universe. While dark matter is comparable to gravity as a pulling force, dark energy is described as “anti-gravity” because it is more of a repulsive force. Dark matter keeps close-ranged galaxies and clusters held together, while dark energy pulls these separate masses apart from each other, stretching the fabric of the universe. If dark matter was not present, dark energy’s forces would make entire galaxies come apart, leaving the universe unfit to sustain any life. Because these two forces are contrasting in nature, you could say that they almost work together to create a balance in how the universe is structured.

Represented by the visual, our current observations of the universe show that its expansion is occuring at an increasing rate, and entire galaxies are being pulled along with it. And before you raise any more eyebrows, yes, it is mildly terrifying. How the universe will end is also attributed to how dark energy will behave in the future, which will be discussed at the very end of the article. Truly, what is science without a few existential crises?

Similarly to dark matter, dark energy is also an invisible force only detected through its gravitational effect. Dark energy repels gravity, which creates an observable distortion in the area around it. To understand how dark energy behaves, we’re going to have to briefly cover Einstein’s gravity theory. Einstein explains gravity as a distortion of space caused by matter or energy. It’s especially relevant in the discussion of dark matter and dark energy, explaining why both distort space/gravity by either attraction or repulsion. On the basis of this theory, scientists have come up with several hypotheses of what dark energy truly is. Their theories are based on mainly one question– what would give dark energy the ability to work against gravity through repulsive forces?

Dark Energy Expansion Visualized

What could it be made of?

Most of the theories surrounding dark energy are based on the concept of “empty space.” This describes areas of space that are essentially rid of any matter, but still contain energy– otherwise known as a vacuum. The majority of our universe is empty space between known bodies of mass, and the amount of empty space continues to increase because of dark energy stretching the universe further apart. Dark energy is emitted from these large areas of empty space, so it must be considered in each prevailing theory on dark energy.

The leading theory for the explanation of dark energy is the cosmological constant theory. The cosmological constant is essentially a measure of the energy that empty space should have to explain the accelerating expansion of our universe. Think of the cosmological constant as the lowest energy state, like a “0” on the scale of energy. Although this form of energy would drive the accelerating expansion that is observed, it still remains a mystery. No one knows exactly why the constant exists in the first place, and the number was actually created by Einstein to fix an error in his theory of general relativity. Dubbed Einstein’s “biggest blunder,” as it turned out to be unnecessary in his calculations, the concept was picked back up again later and utilized for a different purpose when the universe was proven to be expanding at an accelerating rate. What’s even more of a mystery is how the constant would have exactly the right value to cause the observed expansion. The cosmological constant theory is dubbed one of the more “simpler” explanations for dark energy– which I scoff at because it took about an hour to understand the theory well enough to write about it– and has remained the most popular explanation for about two decades.

This theory has also been backed up by evidence recently obtained by researchers at the Baryon Oscillation Spectroscopic Survey (BOSS), who measured galaxies up to 6 billion light-years away to a shocking accuracy of 1%. The measurements corresponded to a form of dark energy that is uniformly spread throughout the universe, which aligns well with the cosmological constant theory and the quintessence theory, which will be discussed later on. The idea of a cosmological constant may seem to contradict the observation of an accelerating universe, but I can assure you that it is consistent with current observations. This is because dark energy’s uniformity in space implies that its effect is constant in terms of time as well. In other words, dark energy’s effect isn’t weakened as time goes on, allowing a constant, gradual expansion of the universe even if its energy was theoretically constant. So, the cosmological constant theory in practice would lead to the acceleration of the universe, but just at a near-constant pace. The rate of acceleration isn't clear from current observations of our universe, and it's likely that the acceleration will occur at either a constant rate (cosmological constant theory) or exponentially (quintessence theory).

The Quintessence theory shares the idea that dark energy is uniform throughout space, but contradicts the cosmological constant theory by claiming that the energy assigned to it isn’t a constant. It assigns the properties of dark energy to an entirely new, fifth force in physics called “quintessence” that changes over time. It can either be an attractive or repulsive force based on the potential to kinetic energy ratio, making it a little bit more nuanced. Because of its ability to fluctuate, quintessence allows for a lot more hypotheticals and a wider range of behavior than the cosmological constant theory. It's possible that quintessence could allow for a very high rate of expansion, or it could potentially fluctuate and cause the universe to slow its expansion, even contracting back into itself at a certain point. I'll further expand on the possible effects of quintessence in the next section.

Other theories range from claiming that dark energy comes from black holes to simply claiming that Einstein’s theory of gravity is untrue, allowing for a wider range of hypotheticals. Because Einstein's theories have been backed up by over a century of research, I didn’t look too deeply into any theories that weren’t grounded in his gravity theory. However, given the amount of limitations in directly studying dark energy, almost anything could be possible.

What is the future of dark matter and dark energy?

Now that we’ve related the two forces of dark matter and dark energy– especially in how they somewhat balance each other out– it’s finally time to get to the more fun questions. Let’s answer my personal favorite: how is the universe going to end?

If you didn’t pick it up from my terrible jokes on existentialism throughout the article, yes, the end of the universe has almost everything to do with dark matter and dark energy. It’s no surprise that these two forces are going to be responsible for the fate of the world, especially because they easily overpower the 5% of our universe that is observable matter. The question then turns into which of the two forces will overpower the other, which can end in multiple different scenarios. Although there are lots of crazy theories out there, I’m only going to be discussing 4 that are clearly linked to dark matter and dark energy interactions.

A Few Helpful Visuals

The big crunch theory describes a universe with a strong dominance of dark and normal matter with a strong gravitational pull. It paints a universe in which matter eventually overpowers the forces of dark energy, and the universe retracts back into itself. The universe has been constantly expanding ever since the big bang, but this theory claims that at some point, matter will overpower the forces of dark energy and will start pulling –or crunching– the universe back into itself. If the quintessence theory were assumed true, then this retraction could alternatively be caused by fluctuations in quintessence.

Although the big crunch theory is pretty interesting, it’s not the most accurate. The most credible theories take into consideration that dark energy is currently overpowering the gravitational forces of dark and observable matter, as expansion of the universe continues to accelerate. For this reason, lots of scientists believe that the big crunch theory is very unlikely to happen. It is still widely discussed, however, because of the quintessence theory of dark energy. Because the properties of quintessence are unknown, we aren’t exactly sure of its behavior in the future. If dark energy was made of quintessence, which can fluctuate over time unlike the cosmological constant, it still remains possible that a big crunch of our universe could happen.

Because dark energy is currently the more dominant force in space, the most popular theories focus more on the effect of accelerating expansion in the long-term. This is true for the accelerating universe theory, which describes a universe that expands infinitely at an exponentially increasing rate due to a strong dominance of dark energy. This theory is very similar in nature to the coasting universe theory, which describes a universe that will expand infinitely at a nearly constant rate due to a slight dominance of dark energy. A coasting universe would be in alignment with the cosmological constant theory of dark energy, while the accelerating universe would be consistent with the quintessence theory of dark energy. In an accelerating universe, quintessence would fluctuate in the opposite manner that it does in the big crunch theory, instead causing a rapid acceleration. Hypothetically, the difference between a coasting universe and an accelerating one could only be determined over a very long period of time. Because we can observe only so much of our universe, it’s hard to tell which theory is true as of now. The ultimate effect of the coasting and accelerating universe theories is referred to as the big freeze, where expansion cools the universe until all matter reaches a uniform state. It would also end in a heat death, where there is no more free thermodynamic energy to sustain motion in our universe.

You’ll be happy to know that I’ve saved the most terrifying theory for last. The Big Rip theory predicts that the prominence of dark energy will be such that in about 22 billion years, the universe will literally tear itself apart, destroying all matter along with it. If dark energy was made up of quintessence, it's entirely possible that its fluctuations could cause it to destabilize in this manner. In theory, the big rip would first destroy galaxies, solar systems, earth, and then finally rip apart atoms themselves.

If it makes you feel better, you and I will be long gone by the time any of these theories have the chance to happen, so I suppose that’s a problem for another day. And you should pat yourself on the back– because you now know almost everything there is to know about not just dark matter and dark energy, but also how the universe could possibly end! So the next time you’re sitting in a class while bored, or perhaps struggling to fall asleep, it’s quite the comforting thought to remember that everyone’s fate is ultimately determined by 2 huge forces that are out of anyone’s control. Enjoy that!

Citations

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