“They Cannot Be Seen Because They Creep Only in the Dark” – Shining a Light on Dark Matter and Dark Energy

It is physically impossible for us to see our entire universe. We face several limitations, the primary one being the finite speed of light. Accounting for the expansion of the universe, it has been estimated that the “observable universe” has a radius of 46 billion light-years, giving a diameter of around 92 billion light years.

This, physically, is our limit. We can only see this part of the universe, which is only a small fraction of the larger universe, which may be 23 trillion light years in diameter, or even infinite. However, in our corner of the universe, now that we have established the limitations, we can then technically see everything, right? It would take some effort, and a lot of time, but all the matter within this region should be visible to us in some capacity. However all the matter that we could possibly see with current methods only comprises 5% of our observable universe. So the question remains, what on Earth (or, in the universe) makes up the other 95%?

The answer to this question is deceivingly simple: dark matter and dark energy, which make up 27% and 68% of the universe respectively. They obtain their names from the fact that, unlike “normal” matter, they are effectively invisible to us when it comes to standard observation methods. Dark matter does not interact with the electromagnetic force, which is the force responsible for light, and thus “seeing”, meaning it cannot be seen directly through conventional methods. In this sense it is invisible, which raises another question. If it cannot be seen, how do we know that it is there?

Figure 1: The Distribution of Energy in the Universe. (Reproduced from Chandra : Resources : Dark Matter (Illustrations), n.d.-b).

The discovery of dark matter came about in an indirect way, just like many other groundbreaking discoveries in the field of physics. When observing the galaxies in our universe, scientists noticed that the galaxies themselves were “achieving an impossible feat”. Their rotation speed was so high that it would be impossible for the matter within the galaxy (the stars, planets, gas clouds, etc.) to hold themselves together under this rotation. The gravitational force between them (which arises from their mass, the more massive a body the greater its gravitational field) would simply be too weak to hold the galaxy together. It is like being on a roundabout, if you were spun around so fast you would struggle to hold on, and be pulled towards the edge. If you were to let go of this roundabout, you would fly off.

According to this observation, these galaxies should just fall apart. All of their components would fly off into the four corners of the universe, and yet this does not happen. The explanation put forward is that there something that we cannot observe directly that is effectively increasing the mass of the galaxies. Something massive (matter) which could not be seen (dark) was thought to be there. And thus “dark matter” was born.

As was mentioned before, dark matter does not interact with the electromagnetic force. Light passes through it, and in a visual sense it is invisible. However, we previously mentioned that the presence of dark matter adds additional mass to a system, increasing the gravitational pull of a galaxy, keeping it together. This is how dark matter is detected, it has a mass, and despite not being visible to our eyes its existence can be observed (or more correctly, inferred) when the gravitational forces seem to be stronger than expected. In these cases, dark matter seems to surround large bodies, like galaxies, in a “halo” of sorts. This provides the galaxy with the mass it needs in order to stay together, like a great cosmic glue.

Dark matter, as a form of matter, clearly has a mass. Otherwise it would not be able to add mass to a system and to subsequently increase the strength of its gravitational pull. But this begs the question, what type of matter is it if it has a mass? It doesn’t seem to be similar to the “normal” matter we deal with, which we call “baryonic” matter. Baryonic matter is made up of baryons, things like protons and neutrons. Scientists are on the search for some type of “dark matter particle”, which represents all of dark matter. This dark matter particle may even lie inside the Standard Model of particle physics (our current understanding of particle physics, which is a list of the variations of subatomic particles and their classifications), just a few steps away from the familiar electron or photon.

However, as of yet there has been little-to-no progress towards a conclusive analysis of dark matter. Part of the difficulty arises because gravity is actually a relatively weak force when compared to the other forces, at least at small scales. For example, at the scale of a proton it is one undecillion times weaker than the electromagnetic force, which is a one followed by 36 zeroes! This means that it is impossible to investigate it in the same way that other particles are investigated (by smashing them together at high velocities in an underground machine in Switzerland), which is even more complicated by the fact that if this was possible, we would still not be able to see the particles.

This has led some researchers to believe that dark matter may not just be a single particle, but instead might be an entire collection of particles, known as the “dark sector”. This would be more than just a family of particles, but a family of forces to govern them, which would, according to the Scientific American, create a “hidden world of cosmology running parallel to our own.” The interactions within this hidden underbelly would shape our observable side of the universe, and may in fact be responsible for many other inconsistencies that we can see. This paradigm shift in the world of dark matter may bring about a breakthrough in our understanding of the universe.

Figure 2: A comparison between our “normal” theory of matter (the standard model) and a hypothetical “dark sector” for dark matter. (Reproduced from Gori, 2020).

It should be noted that until now dark energy has not been mentioned, aside for a brief namedrop at the start of an earlier paragraph. This is because, despite the similar naming scheme, dark matter and dark energy are (as far as we are aware) entirely distinct. However, there is one similarity: they both arose as a hypothetical solution to a problem. Dark matter emerged as a way to explain how a galaxy maintains its structure against all physical odds. Dark energy emerges as a way to explain to accelerating expansion of the universe.

According to Hubble’s Law, the further away an object is, the faster it is moving away from us. However, observations indicate the expansion of the universe has accelerated, and that it will continue to do so. This discovery, which is only about 27 years old, threw a wrench into the conventional view of the universe, which assumed that the expansion would slow down because of the force of gravity, which would cause the original expansion from the big bang (over 13.7 billion years ago) to slow down. This happened, allowing for the formation of stars, but then the expansion somehow sped up again. So, after 13.7 billion years, the universe is not only still expanding, but expanding even faster than before. How is this possible?

The solutions lies in dark energy. It makes up around 68% of the universe, and yet we have very little idea what it is. The man who invented the term even described it as “the most profound mystery in all of science.” Dark energy is much less of a specific term than dark matter, and the only thing more confusing than asking “what is it?” is the question “what isn’t it?”

The current understanding of dark energy suggests that it is some type of energy uniformly distributed across the universe, so that when the universe expands the distribution of this dark energy remains consistent. It is also said to be distributed evenly across space and time, meaning it may be some form of innate energy present within spacetime itself. This energy has some type of repulsive force, which pushes celestial objects (and spacetime itself, it seems) further and further away. This dark energy cannot be observed, just like dark matter. It does not interact with light, and there is no way for us to see it through a telescope or any imaging method. The nature of dark energy itself is shrouded in mystery; it is impossible to describe what it actually is.

Figure 3: The Accelerating Expansion of the Universe due to Dark Energy. (Reproduced from Kazmierczak & Team, 2025c).

There are few clues which can point scientists in a worthwhile direction. It is believed that the prominence of dark energy within the universe is a relatively recent occurrence (on a cosmological timescale, at least), happening around 5 billion years ago. So we have experienced 5 billion years of accelerating expansion of the universe due to some type of energy which we can not only see,  but can’t even identify as being a thing. Frighteningly reminiscent of Kant’s “thing-in-itself”, this dark energy seems to be omnipresent, shaping our perception of the universe by influencing its development, we are doomed to never observe it or experience it directly.

There exist some explanations when it comes to dark energy, describing what it is. It is believed that dark energy, as mentioned before, is some type of energy created by space itself, or in this case the vacuum (emptiness) of space. This is known as the vacuum energy theory, which is actually a creation of Einstein’s, though Einstein was to describe this theory that would define modern cosmology as his “greatest blunder”. Another theory describes dark energy as the result of a field that envelops the entire universe, leading to a variable “value” for dark energy, like an electric field increasing or decreasing in strength. Some go even further, stating that the existence of dark energy proves that there are flaws with our theory of the gravity and the universe. There exists little evidence to prove these theories, but also little evidence to disprove them, due to the mysterious nature of dark matter.

Dark matter and dark energy, named in tandem, are possibly the two greatest mysteries in modern science. Haunting the universe like ghosts, their presence has given rise to our existence, but we cannot observe or interact with them. We are then left with the knowledge that our matter comprises only 5% of the universe, that the other 95% is unseen and unknown.

However, unknown does not mean unknowable. It was only in the previous century that quantum mechanics rose to prominence, and this revelation has unalterably changed the course of modern science. Over the course of a century or more this mystery may begin to unravel thanks to the efforts of generations of scientists. As Newton said: “if I have seen further it is by standing on the shoulders of giants.” Maybe they will see far enough to see dark matter and dark energy, all by standing on our shoulders.

References

Chandra : Resources:  Dark Matter (Illustrations). (n.d.). https://chandra.harvard.edu/resources/illustrations/darkmatter.html

Dark matter. (2025, April 17). CERN. https://home.cern/science/physics/dark-matter

Gori, S. G. (2020, November 20). Probing dark sectors at fixed target experiments using ILC beam [Slide show]. Americas Workshop on Linear Colliders 2020. International Linear Collider : Agenda. https://agenda.linearcollider.org/event/8622/contributions/46550/attachments/35972/55887/TalkILC.pdf

Kazmierczak, J., & Team, N. U. W. (2025, May 2). Dark Matter – NASA Science. NASA Science. https://science.nasa.gov/dark-matter/

Kazmierczak, J., & Team, N. U. W. (2025b, May 2). What is Dark Energy? Inside Our Accelerating, Expanding Universe – NASA Science. NASA Science. https://science.nasa.gov/dark-energy/

Mheslinga. (2025, January 28). Dark energy, explained. University of Chicago News. https://news.uchicago.edu/explainer/dark-energy-explained

Zurek, K. (2025, March 18). ‘Dark Matter’ may be a whole shadow world of mysterious atoms and forces. Scientific American. https://www.scientificamerican.com/article/dark-matter-may-be-a-whole-shadow-world-of-mysterious-atoms-and-forces/