As promised last week, let’s delve into the intriguing subject of dark matter. So, what exactly is dark matter? The answer is somewhat complicated, as dark matter remains a theoretical concept. While not all astronomers agree on its existence, a significant number do believe it represents about 85% of the matter in the universe and roughly 27% of its total mass-energy density. According to the standard cosmological model, a mere 5% of the universe's total mass-energy is composed of regular matter and energy, with the remaining 68% attributed to a different energy form called dark energy.

However, this does not clarify what dark matter is composed of. The reality is that scientists are still unsure about the exact nature of dark matter, rendering it quite mysterious! Various hypotheses exist regarding its composition, but to grasp these theories, one must first understand what we refer to as regular matter. Regular matter, or baryonic matter, constitutes everything we see on Earth, from the stars above to the galaxies scattered throughout space. It consists of particles such as protons, neutrons, and electrons. Protons carry a positive charge and, along with the neutral neutrons, form the nucleus of atoms. The quantity of protons and neutrons in this nucleus determines the type of atom formed, with negatively charged electrons orbiting around the nucleus. This foundational knowledge is likely familiar to you, so I won't elaborate further.

One hypothesis regarding the composition of dark matter suggests it could be regular matter that is simply more difficult to detect with our current technology. Possible candidates for this include dim brown dwarfs, white dwarfs, and neutron stars. Supermassive black holes might also contribute to this missing mass. However, these elusive entities would need to account for a more significant portion of the universe's mass than currently observed, leading many scientists to propose that dark matter is likely composed of non-baryonic matter, which does not consist of protons and electrons but rather unknown particles.

The primary candidate for these particles is WIMPs, or weakly interacting massive particles. These particles are termed "weakly interacting" because they have minimal influence on their surroundings. Although WIMPs remain entirely hypothetical, they are generally defined as superheavy particles that exert gravitational pull on both baryonic and non-baryonic particles while rarely interacting otherwise. Many theorists believe WIMPs were thermally produced in the early universe, akin to baryonic matter particles, and are expected to constitute cold dark matter, meaning they would move at very slow speeds.

Detecting WIMPs is challenging since they interact through only gravitational and weak forces. Nevertheless, numerous experiments are currently underway to identify WIMPs, both directly and indirectly. Indirect detection involves observing WIMPs at significant distances from Earth, focusing on areas where WIMP dark matter is believed to accumulate, such as the centers of galaxies and galaxy clusters, as well as the smaller satellite galaxies of the Milky Way. These regions are particularly advantageous as they contain minimal baryonic matter, thereby reducing the expected background noise from standard astrophysical processes. Typically, indirect searches aim to find excess gamma rays produced when charged WIMP particles interact.

Conversely, direct detection seeks to observe the effects of a WIMP collision as dark matter interacts with a detector situated in an Earth-based laboratory. While most WIMPs passing through the Sun or Earth are anticipated to do so without any noticeable effect, scientists hope that a sufficiently large detector will register enough interactions to yield a few detectable events each year. Current methods to detect WIMPs emphasize finding highly sensitive systems that can be scaled to larger volumes.

Another potential candidate for dark matter particles is Massive Compact Halo Objects, abbreviated as MACHOs. Although they have a contrasting name to WIMPs, their definitions share similarities. MACHOs are bodies that emit little or no radiation and drift through interstellar space without being part of any planetary system. They could consist of normal baryonic matter or not. Due to their non-luminous nature, MACHOs are also challenging to detect. They may be identified through gravitational microlensing, where their gravity bends the light from a star behind them, making the star appear brighter. Several research groups have searched for MACHOs by studying microlensing effects, but results have been largely inconclusive.

Sterile neutrinos represent another proposal for dark matter particles. Neutrinos are elusive subatomic particles produced in various nuclear processes and do not constitute regular matter. Their name, meaning "little neutral one," highlights their lack of electrical charge. While a constant stream of neutrinos from the Sun passes through Earth undetected due to their rare interactions with normal matter, sterile neutrinos are theorized as a potential dark matter candidate, interacting only through gravitational forces. Regular neutrinos possess a non-zero mass, meeting some criteria for dark matter particles. If sterile neutrinos do not emit electromagnetic radiation, they could indeed represent dark matter.

Many theories regarding dark matter's composition have been rejected over time. One such idea proposed that primordial black holes—black holes formed in the early universe with masses exceeding 30 times that of the Sun—could generate dark matter. In 2015, gravitational waves from a primordial black hole merger were detected, indicating an event from the universe's infancy. Scientists anticipated observing at least eight supernova explosions resulting from the merger's energy, but none were reported. Despite particles being expelled or absorbed during the merger, subsequent studies concluded that this theory could not account for dark matter, especially after further analysis of the sun’s heliosphere suggested energy levels too low to support it.

You may have noticed the mention of "cold dark matter." This raises the question: does hot dark matter exist? Indeed, hot dark matter is a scientific concept, along with warm dark matter. These categories classify dark matter based on particle velocity rather than temperature, reflecting the distances particles moved during the universe’s early stages before cosmic expansion slowed them down—a crucial distance referred to as the free streaming length. Scientists can backtrack using the universe's expansion rate to determine the initial velocities of these particles.

The classification hinges on the size of a protogalaxy, which evolves into a dwarf galaxy. Dark matter particles are categorized as cold, warm, or hot based on their free streaming lengths relative to the protogalaxy size. Cold dark matter particles are significantly smaller, warm dark matter is similar in scale, and hot dark matter particles exceed protogalaxies in size. Mixtures of these types are also theoretically possible, though a mixed dark matter model was largely abandoned after the discovery of dark energy.

Cold dark matter leads to a bottom-up structure formation, where galaxies form first, followed by clusters, while hot dark matter would result in a top-down formation, where large aggregations form first and later split into separate galaxies. Current understanding of galaxy and supercluster formation suggests that galaxies likely formed first, which is why cold dark matter is widely supported. Furthermore, cold dark matter is considered the simplest type of dark matter particle. Initially, scientists entertained the possibility of warm dark matter, but a lack of viable candidates led to a preference for cold dark matter.

If dark matter consists of weakly interacting particles, one might wonder if it can form structures akin to planets, stars, or black holes. Historically, the consensus has been that it cannot, due to two primary reasons. First, dark matter lacks a mechanism to efficiently lose energy, which is vital for structure formation. Regular matter can form dense objects because it possesses various ways to dissipate energy. If dark matter particles were to gain energy during compaction under gravity without a means to lose it, they would heat up and accelerate away from forming structures.

Second, dark matter lacks the range of interactions necessary for structure formation. Ordinary matter can interact in multiple ways, facilitating the development of complex structures. For example, stars initially form through gravity, but particle interactions enable energy emission through processes like fusion. Dark matter appears limited to gravitational interactions, lacking evidence of the diverse interactions exhibited by ordinary matter.

Another pressing question surrounds how scientists confirm the existence of dark matter. If it is so elusive, does it exist at all? While this remains a possibility, various astrophysical observations imply its presence, such as gravitational effects that existing theories cannot explain without accounting for unseen mass. Consequently, most experts believe dark matter is prevalent in the universe and significantly influences its structure and evolution. Dark matter is termed "dark" because it does not interact with the electromagnetic field, making it nearly impossible to detect.

Key evidence for dark matter arises from calculations indicating that many galaxies would not remain intact or exhibit their observed movements without substantial unseen mass. Other supporting observations include gravitational lensing and the cosmic microwave background (CMB)—the residual radiation from the universe's inception. The CMB originates from approximately 400,000 years post-Big Bang, when the universe was cool enough for hydrogen to form, allowing CMB photons to travel in straight lines with minimal interference.

Approximately 380,000 years after the Big Bang, the universe had expanded sufficiently that matter could no longer obstruct CMB photons. Mapping the CMB allows us to look back in time, revealing fluctuations in visible matter and a peculiar "cold spot" near the equator lacking visible matter—suggesting the presence of dark matter.

Additionally, the rotation curves of galaxies support the existence of dark matter. As spiral galaxies rotate around their centers, the density of visible mass decreases outward. If luminous mass were the sole component, we would expect a decrease in rotation velocity with distance from the center, akin to our Solar System. However, observations reveal that the rotation speed of spiral arms actually increases with distance from the center, indicating a significant amount of non-luminous, or dark, matter in the outskirts.

Despite this wealth of evidence, dark matter remains a hypothetical entity because scientists have yet to detect it directly. They can deduce its existence, much like inferring the temperature outside during winter without stepping outside, but definitive proof remains elusive.

This raises the question of detection methods for dark matter. Scientists employ two primary approaches: direct detection and indirect detection. Direct detection experiments aim to observe low-energy recoils of dark matter particles interacting with Earth's atmosphere. If dark matter exists, some particles should enter our atmosphere and interact with other particles, leading to detectable energy transfers. Numerous observatories are developing highly sensitive detectors to capture these interactions, but background noise from other particle interactions complicates the task of isolating dark matter signals.

Consequently, much of the effort focuses on indirect detection, where scientists search for decay products of dark matter particles. Particle decay, the spontaneous transition of one particle into lower-energy particles, is anticipated to occur with dark matter particles as well. This decay could release energy into the universe, indirectly contributing to an increase in antimatter in specific regions. If scientists can measure this increase without other plausible explanations, dark matter is likely involved. However, this method also faces challenges, as various astrophysical sources can mimic expected signals, necessitating multiple observations for confirmation.

Some researchers have shifted their focus away from detection and are attempting to recreate dark matter in laboratories. Experiments at the Large Hadron Collider, a massive particle collider, could potentially detect dark matter particles generated through proton beam collisions. If dark matter particles exhibit minimal interactions with ordinary matter, they may be inferred through missing energy and momentum in detection systems, provided other detectable collision products are present.

Given that dark matter remains hypothetical, alternative theories have emerged. One prevalent notion involves modifying general relativity, the geometric theory of gravitation established by Albert Einstein in 1915. While general relativity has consistently aligned with experimental observations on smaller scales, its validity on a universal scale is still debated. Some scientists propose that minor modifications to general relativity could eliminate the need for dark matter, thus alleviating the ongoing search. However, altering general relativity introduces new challenges, as the modified theory must still account for gravitational movements of celestial bodies, which general relativity currently explains effectively. With ample indirect evidence supporting dark matter's existence, it seems likely that it is indeed a reality.

To conclude this exploration of dark matter, let's reflect on its historical context. In modern times, astronomer Fritz Zwicky first observed what we now recognize as dark matter in the 1930s. His 1933 observations of the Coma Cluster of galaxies suggested a mass 500 times greater than that calculated by Edwin Hubble, with this extra mass appearing completely invisible. Initially met with skepticism, Zwicky's findings were later corroborated by other astronomers.

Three decades later, astronomer Vera Rubin provided compelling evidence for dark matter's existence by demonstrating that the centers of galaxies rotate at the same speed as their outer regions, contrary to expectations. This observation implies that galaxies are only the visible portions of much larger structures, akin to labels on a vinyl record, causing consistent rotation speeds throughout.

Rubin’s findings, building on Zwicky's work, faced resistance within the astronomical community but ultimately became pivotal proof of dark matter's existence. Today, scientists continue to diligently search for concrete evidence of this enigmatic substance. The timeline for such a discovery remains uncertain, but it is widely believed that dark matter is out there.

Thank you for joining me in this discussion on dark matter. While numerous mysteries still surround this material, I hope today's episode has illuminated some of them. Next week, we will explore another enigma of our universe: dark energy, which is thought to make up the majority of the cosmos. Be sure to tune in for that!