What isn’t Dark Matter?

Something that is an ordinary matter.

6 min readAug 12, 2023

Imagine standing beneath a starlit sky, gazing up at the constellations that have captivated human beings for millennia. We’re not alone in our curiosity about the cosmos; astronomers, too, have been peering into the heavens, seeking answers to some of the universe’s most perplexing mysteries. One such conundrum that has sparked their curiosity is the existence of dark matter — a form of matter that eludes our senses but exerts its gravitational influence throughout the cosmos. So, what exactly is this elusive cosmic entity, and how have astronomers been trying to uncover its identity? Let’s embark on a journey of cosmic detective work as we uncover the secrets of dark matter.

The Game of Elimination: Ruling Out Suspects

In the cosmic realm, astronomers have been playing a cosmic game of elimination, much like a detective in pursuit of a cunning culprit. They’ve been ruling out potential suspects one by one to get closer to the truth. As we delve into this intriguing mystery, let’s explore some of the key suspects that have been considered and subsequently ruled out in the quest to unmask dark matter.

Black Holes: A Bitter Farewell to the Easiest Suspect

At first glance, black holes might seem like the perfect candidate for dark matter — enigmatic, invisible, and ominously dark. But this is where our cosmic detective skills come into play. Black holes, while indeed elusive, can’t escape their past. Whenever a black hole forms, it leaves a trail of destruction in its wake, often marked by violent explosions known as supernovae. These cataclysmic events generate high-energy emissions and X-rays that linger for millennia. So, if the universe were filled with enough black holes to account for dark matter, the aftermath of these stellar parties would be impossible to ignore. Yet, our cosmic scene remains devoid of such evidence, rendering black holes an improbable candidate for the dark matter mystery.


Massive Compact Halo Objects (MACHOs): A Substellar Sleuth

In our cosmic investigation, we venture further into the shadows, exploring a realm of collapsed objects known as MACHOs — Massive Compact Halo Objects. These objects, which range from sub-stellar to Jupiter-like masses, could be hiding within the vast cosmic halo that envelops our galaxy. However, gravitational lensing, a phenomenon where mass bends light, cast doubt on the MACHO theory. Experiments conducted in the late 1980s looked for instances of gravitational lensing throughout our galaxy’s halo. While MACHOs could potentially create lensing effects, the data painted a different picture. The statistics from these experiments discredited the notion that MACHOs constituted the majority of dark matter.

Massive Compact Halo Objects

From Asteroids to Dust Grains: A Cosmic Cul-de-sac

As our cosmic detective work continues, we turn our attention to even smaller suspects — objects ranging from asteroid size down to dust grain size. Could these tiny entities be the culprits behind dark matter’s enigma? The interstellar medium and the spaces between planets are indeed filled with rocks, dust, and particles. However, their cover is blown by infrared radiation. Objects like boulders, asteroids, and even house-sized remnants emit infrared radiation due to the light they absorb from neighboring stars or galaxies. The Infrared Astronomical Satellite (IRS) mission conducted by NASA some decades ago looked deep into the cosmos in far infrared wavelengths. This sensitive mission should have detected the faint glow of radiation from these hypothetical objects if they existed. Yet, the data told a different story — the objects were nowhere to be found.

On the Trail of Subatomic Secrets

A hidden force that shapes the cosmos yet evades detection, it’s a puzzle that has stumped astronomers and physicists alike. As we journey deeper into the cosmic abyss, we uncover the current frontrunner in the quest to unmask dark matter’s true identity: the mysterious subatomic particle.

The Lone Contender: Subatomic Particle in the Spotlight

Imagine a puzzle with all the pieces scattered across the table, waiting for that final piece that completes the picture. Dark matter’s mystery is a bit like that — a cosmic conundrum with a multitude of theories, all seeking to fill in the missing gaps. Yet, after meticulously scrutinizing and ruling out a host of possibilities, only one contender remains the subatomic particle.

This particle, if it exists, must play a dominant role in the universe, surpassing the number of familiar particles like protons, neutrons, and electrons by a significant factor. It’s an omnipresent entity, yet it must interact ever so faintly with electromagnetic radiation — a condition that narrows down the options considerably. These particles are dubbed “weakly interacting massive particles,” or WIMPs for short.

A Convergence of Theories: Supersymmetry Enters the Stage

Within the realm of theoretical physics, there exists a tantalizing extension of the Standard Model — the blueprint that describes the fundamental particles and forces in the universe. This extension is called supersymmetry, and it proposes a symmetrical relationship between particles with half-integer spin (fermions) and those with integer spin (bosons). Essentially, each particle type would have a shadowy counterpart.

Supersymmetry might be the key to uncovering dark matter’s mystery. The lightest supersymmetric particles could possess the qualities necessary to be the elusive WIMPs that constitute dark matter. It’s an enticing connection between the desire of physicists to expand the Standard Model and astronomers’ quest to explain dark matter.

The Quest for Validation: Seeking Dark Matter in Labs and Colliders

However, the path to confirming supersymmetry and dark matter’s connection is far from straightforward. High-energy physicists are conducting experiments at the Large Hadron Collider (LHC) in CERN to uncover the lightest supersymmetric particles. These particles, if they exist, could be prime candidates for dark matter. Yet, the journey is fraught with uncertainty.

Recent updates from CERN have provided both hope and disappointment. While hints of supersymmetry have not surfaced in the LHC’s high-energy collisions, this doesn’t definitively rule out the theory. Supersymmetry, it turns out, isn’t a single theory but a collection of potential explanations, each with its unique predictions.

Deep Underground: The Hunt for Weakly Interacting Massive Particles

In the quest for dark matter, researchers have descended deep into Earth’s embrace, seeking a quiet environment free from the interference of cosmic rays. These experiments involve shielding ultra-pure solid-state materials like silicon or germanium from cosmic noise. The goal is to patiently observe the detectors for any rare interactions with passing dark matter particles.

The challenge lies in the particle’s weak interaction strength, which necessitates long periods of observation. Researchers are dedicatedly engaged in these experiments, tirelessly waiting for the faintest sign of a dark matter particle revealing its presence.

In Conclusion

The pursuit of understanding dark matter has taken astronomers and physicists on a captivating journey of cosmic detective work. Through a process of elimination, various potential suspects, from black holes to massive compact halo objects and even small entities like asteroids and dust grains, have been ruled out as the primary source of dark matter. The current frontrunner in this enigma is the subatomic particle, specifically weakly interacting massive particles (WIMPs), which are theorized to play a dominant role in the universe while interacting only faintly with electromagnetic radiation.

The concept of supersymmetry, an extension of the Standard Model in theoretical physics, presents a promising avenue for explaining dark matter. Supersymmetry suggests a relationship between particles with different spins, and the lightest supersymmetric particles could potentially be the elusive WIMPs constituting dark matter. Despite the uncertainty and challenges faced in validating this theory, experiments conducted at the Large Hadron Collider (LHC) and deep underground laboratories continue to search for evidence of these particles.

As the cosmic conundrum of dark matter persists, researchers remain committed to unraveling its secrets, pushing the boundaries of human knowledge and exploration to shed light on one of the universe’s most perplexing mysteries.