Our universe is compromised of several matters and these matters, collectively, create our universe and shape how it works. Antimatter, dark matter, and normal matter, are the three building blocks of matter that exist in our universe. At least, the ones that we are aware and can understand right now. Among these, dark matter is the second most frequent matter in our universe. 26.8% of our universe is made up of dark matter. It exists so much but what is dark matter?

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Unfortunately, really understanding dark matter is one of the greatest mysteries in the field of astrophysics. Many astrophysics start their journey with dark matter with the simple question I asked above, “what is dark matter?” Because we don’t even exactly know what it is. It’s highly mysterious, like many other matters in the universe. We are still yet to understand this concept fully. Despite its elusive nature, scientists have been studying it for decades. Let’s take a look at what dark matter is, what exactly it does, and what is the nature around it.

Introduction to Dark Matter

The exact definition of dark matter might not be easy and too detailed for the average astronomy lover. However, it is crucial to understand what it is, no matter how complicated it might be. We first need to have a basic understanding of the composition of the universe.

The universe is made up of various components, including ordinary matter, which consists of atoms and subatomic particles. This ordinary matter, such as stars, planets, and gas clouds, is what we can observe and interact with directly. However, with more observations and calculations we realized that the visible matter in the universe only accounts for a small fraction of its total mass. Most of the universe is composed of something invisible and mysterious, which we named dark matter.

This matter does not emit, absorb, or reflect light. This makes it nearly impossible to detect using traditional telescopes or other optical instruments. We prove its existence through its gravitational effects on visible matter and by observing the structure of the universe. Even though dark matter is invisible, its influence is quite big because it effects through gravitation. It has a crucial role in the formation and evolution of galaxies, the large-scale structure of the universe, and the patterns observed in the cosmic microwave background radiation.

As I mentioned, we can currently study dark matter indirectly through its gravitational effects on the other matter in the universe – like to the planets, stars, and other cosmic stuff. This is a good starting point but if we want to undrstand more of dark matter, we need to find a way to directly detect it, not indirectly.

The Discovery and Evidence of Dark Matter

The existence of dark matter was first proposed in the 1930s. The first one to mention the existence of it was Swiss astronomer Fritz Zwicky. While studying the Coma Cluster of galaxies, Zwicky noticed that the visible matter in the cluster wasn’t sufficient to explain the high velocities of the galaxies within it. He hypothesized the presence of unseen matter, which he referred to as “dunkle Materie”, which is the original German name of dark matter. Since Zwicky’s initial proposal, there have been a lot of studies with evidence to support it’s existence. It first started to came in around 1970s with Vera Rubin and Kent Ford.

Historical Background of Dark Matter

The pioneering work of Vera Rubin and Kent Ford in the 1970s provided compelling evidence. They studied the rotational speeds of stars within galaxies. They found that we can’t explain observed velocities solely by the visible matter. The presence of unseen mass, which is the dark matter, was necessary to account for their observations. Basically, Vera and Kent saw that there is an emptiness that we can’t explain with the visible matter that we know. There has to be something else, which is the dark matter.

The Bullet Cluster, observed in 2006 by the Hubble Space Telescope and the Chandra X-ray Observatory, provided even more evidence. These two galaxy clusters collided and this collision revealed a separation between the visible matter (gas and stars) and the gravitational mass distribution, indicating the presence of dark matter.

Astrophysical Evidence and Observations

• Large-scale structure formation: Dark matter’s gravitational pull influences the distribution of galaxies and galaxy clusters across the universe. The observed patterns of large-scale structures provide evidence for its the existence.
• Cosmic microwave background radiation: The cosmic microwave background (CMB) radiation is the residual heat from the early universe. Precise measurements of the CMB, such as those obtained by the Planck satellite, have revealed fluctuations that are consistent with the presence of dark matter.

The Role of Gravitational Lensing

Gravitational lensing is when the gravitational field of a huge object, like a galaxy or a galaxy cluster, bends the path of light from more distant objects. The gravitational effects of dark matter can cause additional lensing, enabling scientists to map the distribution of dark matter in the universe.

We managed to observe the phenomenon of gravitational lensing in various contexts. These observationsp provided an indirect evidence, showing that it can exist. Examples include the observation of multiple images of distant galaxies and the measurement of mass distributions in galaxy clusters. The cumulative evidence from these observations and studies strongly suggests that dark matter is an essential component of the universe. Its gravitational effects shape the structure of galaxies and the large-scale distribution of matter.

Characteristics and Composition of Dark Matter

I already told you how dark matter is invisible and that is not easy to study it because of this. However, we still managed to do a lot of things to understand it as much as we can. Now, we have a set of characteristics and composition of it.

Distinguishing Dark Matter from Ordinary Matter

• Dark matter does not interact with electromagnetic radiation, which includes light and other forms of electromagnetic waves. This property makes it challenging to detect using traditional telescopes and optical instruments.
• Unlike ordinary matter, dark matter does not experience electromagnetic forces, such as electromagnetic repulsion or attraction. This characteristic allows it to pass through ordinary matter without any significant interactions.
• We believe dark matter to be non-baryonic, meaning it is not composed of the same building blocks as ordinary matter (protons, neutrons, and electrons). This distinction further sets it apart from ordinary matters.

Possible Types of Particles

• The leading candidate for dark matter particles is the Weakly Interacting Massive Particles (WIMPs). WIMPs are hypothetical particles that interact weakly with ordinary matter and have significant mass. Several experiments are underway to directly detect WIMPs through their rare interactions with ordinary matter.
• Axions are another proposed class of particles. These hypothetical particles are extremely light and have unique properties that make them difficult to detect. The search for axions is an active area of research in the field of dark matter.
• Other candidates include sterile neutrinos, gravitinos, and superpartners (predicted by supersymmetry). These particles have different properties and hypothetical interactions, making their detection and identification challenging.

It’s The Role in the Universe

• Dark matter has a vital role in the formation and evolution of galaxies. Its gravitational pull helps bind galaxies together and enables the formation of large-scale structures, such as galaxy clusters and superclusters.
• The presence of it explains the observed rotational curves of galaxies. These curves show that stars and gas in galaxies rotate at speeds that cannot be accounted for by the visible matter alone. The additional mass helps explain this phenomenon.
• Dark matter also influences the cosmic microwave background radiation, leaving imprints in its temperature and polarization patterns. By studying these imprints, scientists gain insights into the abundance and properties of it in the early universe.

Theories and Models Related to Dark Matter

In the quest to understand the nature of this matter, scientists have developed various theories and models that attempt to explain its existence and properties. These theories may be the light to understand it more in detail and take a few steps ahead to detect it directly. The main reason why we call them theories or even these “theories” comes up is because our information is limited. We need to understand as much as we can about dark matter and the good way to start is with theories.

Cold Dark Matter Theory

The Cold Dark Matter (CDM) theory is one of the most widely accepted and studied models. According to this theory, this matter consists of slow-moving particles that formed shortly after the Big Bang. These particles, known as cold dark matter particles, interact weakly with ordinary matter and have substantial mass. The CDM theory successfully explains the large-scale structure formation observed in the universe, as well as the observed rotational curves of galaxies. It predicts the formation of dark matter halos, which provide the gravitational framework for galaxy formation.

Simulations based on the CDM theory have been able to reproduce observed cosmic structures, such as galaxy clusters and filaments, with remarkable accuracy. However, challenges remain in explaining certain observations at smaller scales, such as the distribution of dwarf galaxies.

Warm Dark Matter Theory

The Warm Dark Matter (WDM) theory proposes that dark matter particles have intermediate masses and velocities between those of cold and hot dark matter. WDM particles would still be moving relatively slowly but with higher velocities compared to CDM particles. The WDM theory aims to address some of the challenges faced by the CDM theory at smaller scales. It suggests that the presence of WDM would suppress the formation of small structures, such as dwarf galaxies and low-mass galaxy clusters.

While the WDM theory offers an alternative perspective, it is still an area of ongoing research and debate. Scientists are actively exploring ways to test and distinguish between the predictions of the CDM and WDM theories.

Alternatives to Dark Matter Theories

In addition to the CDM and WDM theories, several alternative theories and models have been proposed to explain the observed phenomena. These include Modified Newtonian Dynamics (MOND), which suggests modifications to the laws of gravity at low accelerations, and emergent gravity theories, which propose that gravity emerges as an emergent phenomenon from the collective behavior of ordinary matter.

These alternative theories aim to explain the observed effects attributed to dark matter without invoking the existence of new particles. While they have gained attention and sparked debates within the scientific community, they have yet to gain widespread acceptance due to various challenges and limitations.

Current Research and Unanswered Questions

Current research efforts focuses on addressing the numerous unanswered questions surrounding this mysterious substance. Despite significant advancements, there are still many aspects that remain elusive.

Direct Detection

• Direct detection experiments aim to directly observe the interactions between dark matter particles and ordinary matter. These experiments typically involve highly sensitive detectors placed deep underground to shield from background radiation.
• We employ various various detection techniques, such as scintillation, cryogenic, and superconducting detectors to search for the rare signals associated with dark matter interactions. Several experiments, including XENON, LUX-ZEPLIN, and DarkSide, are actively searching for direct evidence.
• To date, we never made conclusive direct detection of dark matter particles. This places constraints on the properties and interactions of dark matter. Ongoing efforts continue to refine experimental techniques and increase sensitivity to increase the chances of detecting it directly.

Indirect Detection Methods

• Indirect detection methods involve searching for the products or signals resulting from the destruction or decay of dark matter particles. These methods rely on observing high-energy particles, such as gamma rays, neutrinos, or cosmic rays. These could be produced in the interactions.
• Observatories like the Fermi Gamma-ray Space Telescope and the High-Altitude Water Cherenkov Observatory (HAWC) are dedicated to detecting potential signals of dark matter annihilation or decay.
• Indirect detection methods also include searching for the effects of this matter in astrophysical phenomena, such as cosmic rays, high-energy neutrinos, or gamma-ray emissions from dwarf galaxies or galaxy clusters.

Unanswered Questions and Future Prospects

• The nature and properties of dark matter particles remain unknown. Identifying the specific particles or particles responsible for dark matter is a fundamental question in astrophysics and particle physics.
• We still need to fully understand the distribution of dark matter within galaxies and galaxy clusters, as well as its interaction with ordinary matter. How dark matter behaves on small scales and its role in galaxy formation and evolution are active areas of research.
• The connection between dark matter and other fundamental particles and forces in the universe, such as the Higgs boson or neutrinos, is an intriguing avenue of investigation.
• Exploring alternative theories and models to explain the observed phenomena attributed to dark matter is an ongoing pursuit. Investigating modified theories of gravity and emergent gravity theories can provide alternative perspectives and potential solutions.

As we continue to research, develop new technologies, and create new observational techniques, there is still hope to be able to shed light on this matter’s mysteries and how we can detect it, study it, and understand it more. The more we answer all these unanswered questions I just discussed above, we keep the ability to be able to uncover the real truth and nature of dark matter. However, this is going to take a while and it probably wont happen in our lifetime.