The Invisible Architect of the Universe

Part 1: The Great Question Imprinted on the Universe

The Mystery Named 95%

Did you know that all the stars we see in the night sky, the planets we stand on, and all the matter including ourselves make up only 5% of the entire universe? It’s like entering a huge library and thinking you know everything just by reading the table of contents of one book. The biggest question modern cosmology poses to us is this: “Then what exactly is the remaining 95%?”

Scientists have labeled these invisible entities. About 27% is a cosmic glue called ‘dark matter’ that holds the universe together, and about 68% is an unknown force called ‘dark energy’ that causes the universe to expand faster and faster.

The word ‘dark’ might make you think of something black and evil, but it actually means ‘unknown.’ These entities neither emit, reflect, nor block light—they simply let it pass through, making them literally ‘transparent beings’ invisible to our eyes or any instruments.

This story is about detectives tracing the traces of the invisible architect, dark matter, which designed the vast structure of the universe and formed the galaxies we see today.

The Ghost’s First Whisper: Searching for Evidence

Scientists became convinced of the ghostly presence of dark matter because of consistent whispers heard from different times and places.

The Spinning Carousel That Turns Too Fast: Galaxies

The first whisper was about the rotation speed of galaxies. In the 1970s, astronomer Vera Rubin discovered something strange. Just as planets farther from the Sun in our solar system orbit more slowly, stars in a galaxy should also orbit more slowly the farther they are from the center. But the galaxies she observed had outer stars rotating as fast as those near the center, as if something invisible was holding them tightly.

The only way to solve this mystery was the hypothesis that a huge ‘dark matter halo’ envelops the entire galaxy far beyond the visible stars and gas. This invisible mass, five times greater than visible matter, acts like a hand holding the giant rotating carousel of the galaxy together.

The Magnifying Glass That Warps Spacetime: Gravitational Lensing

Einstein said mass warps the surrounding spacetime, like a heavy bowling ball pressing down on a rubber sheet. Light passing through this curved space also bends. This is the ‘gravitational lensing effect,’ and importantly, it happens regardless of whether the mass emits light or not. Thanks to this, we have a powerful tool to ‘see’ the presence and distribution of dark matter, which does not interact with light.

Sometimes, light from distant galaxies appears stretched into long arcs, and by statistically analyzing the slight distortions of galaxies across the universe, we can create a 3D map showing where and how much dark matter is clustered. This map revealed that dark matter is spread like a gigantic cosmic web throughout the universe.

The Universe’s First Cry: The Primordial Light

When the universe was about 380,000 years old, it cooled enough for light to travel freely. This ‘primordial light’ has reached us today as the ‘Cosmic Microwave Background (CMB).’ This light is like a baby photo of the entire universe.

This baby photo shows tiny temperature fluctuations forming patterns. By analyzing these patterns, we can learn what the universe is made of. The results were astonishing: these patterns could not have formed unless dark matter, which does not interact with light, was five times more abundant than ordinary matter. Dark matter was already there shortly after the universe was born.

The Skeleton of the Giant Architecture: The Cosmic Large-Scale Structure

Today, galaxies are not scattered randomly but distributed along a cosmic web of massive filaments called the ‘cosmic large-scale structure.’ How was this huge structure formed?

Right after the Big Bang, ordinary matter could not clump easily due to radiation pressure from light. At this time, dark matter, unaffected by light, began to clump first, digging ‘gravitational wells.’ Like building a structure by erecting a framework (scaffold) first, later as the universe cooled, ordinary matter was drawn into this dark matter framework, finally forming stars and galaxies.

These four pieces of evidence, from different eras and places, all point to the same suspect: dark matter. Now, we scientists, as detectives, embark on a full-scale investigation to reveal the identity of this ghost.

Part 2: The Story of Ghost Hunters

Faced with strong evidence for dark matter’s existence, the next question scientists naturally asked was: “So, what exactly is it?” To answer this, scientists worldwide launched a massive ghost-hunting operation.

Leading Suspects

Dark matter cannot be ordinary particles we know. If it were, we would have discovered it already. So scientists think it must be a new particle unknown to us and have shortlisted some leading suspects.

WIMP (Weakly Interacting Massive Particle)

For decades, the most promising suspect was the ‘WIMP,’ meaning a ‘weakly interacting massive particle.’ WIMPs are thought to be tens to thousands of times heavier than protons and interact only via very weak forces.

WIMPs were special because of the so-called ‘WIMP miracle.’ Complex calculations showed that if particles like WIMPs existed in the early universe, as the universe cooled, just the right amount of dark matter we observe today would remain naturally. The fact that a particle predicted by a completely different theory perfectly explains the universe’s mystery excited scientists greatly.

Axion

However, after decades of searching for WIMPs with no success, a new suspect emerged: the ‘axion.’ Unlike WIMPs, axions are extremely light particles, trillions of times lighter than electrons.

Axions are attractive because, like WIMPs, they were not originally proposed to explain dark matter. Axions were suggested to solve another puzzle in particle physics, but it turned out they perfectly satisfy the conditions for dark matter. It’s like making a house key that also opens a bank’s secret vault.

A Global Investigation Network

To catch these suspects, scientists cast a huge net using three different methods:

1. Direct Detection (Waiting Trap): This method tries to catch the rare moment when dark matter particles passing through our galaxy collide with detectors deep underground. These collisions happen only a few times a year, so scientists create extremely quiet environments that block all other noise to listen for dark matter’s whispers.

2. Indirect Detection (Looking for Leftover Traces): When two dark matter particles collide and annihilate, they may leave traces like gamma rays. This method uses space telescopes to observe places rich in dark matter, like galaxy centers, searching for these traces (smoke) to track down the culprit (fire).
3. Collider Searches (Creating the Culprit): Instead of waiting or looking for traces, this approach tries to recreate the conditions just after the Big Bang to produce dark matter directly. Facilities like the Large Hadron Collider (LHC) smash particles at tremendous speeds to find evidence of dark matter as invisible particles (missing energy) created in the collisions.

Part 3: The Unfinished Debate

The idea of dark matter has been very successful in explaining the universe but has not explained everything perfectly. Some observations contradict existing ideas and have sparked new debates.

Small Cracks in the Standard Model

Dark matter simulations predicted a sharp peak (’cusp’) in the density at galaxy centers, but many observed galaxies show a flat ‘core’ structure instead. Also, simulations predicted hundreds of small satellite galaxies around our Milky Way, but only dozens have been found.

Is this ‘small-scale crisis’ evidence that the dark matter theory is wrong? Not necessarily. It might be that energy from star formation and explosions (baryonic feedback) affected dark matter distribution, flattening the centers. Or it might hint that dark matter has slightly different properties than we thought.

No Ghost? New Gravity Theories

“What if the ghost called dark matter doesn’t exist at all?” This bold idea gave rise to the theory called ‘Modified Newtonian Dynamics (MOND).’ Instead of assuming unknown matter, it claims that gravity behaves differently under very weak forces than we have known.

MOND surprisingly explains the rotation speeds of many galaxies accurately without dark matter. However, it fails to explain larger structures like galaxy clusters or the evolution of the entire universe, hitting its limits.

Decisive Evidence: The Bullet Cluster

A decisive piece of evidence that settled the long debate between dark matter and MOND was found in the ‘Bullet Cluster.’ This is a cosmic traffic accident where two massive galaxy clusters collided head-on.

During this collision, something remarkable happened:

• Ordinary Matter (Hot Gas): The gas, which makes up most of the cluster’s mass, collided and slowed down, remaining near the center (pink in the image).
• Stars and Galaxies: Passed through each other almost unaffected.
• Center of Total Mass: Measured by gravitational lensing, the mass center did not coincide with the ordinary matter but matched the positions of the galaxies that passed through (blue in the image).

This means the majority of the mass (gas) and the center of gravity are separated. This is strong evidence that the source of gravity is something invisible and non-interacting that moved with the galaxies. This phenomenon cannot be explained by MOND and is considered the most direct proof of dark matter’s existence.

Part 4: Exploring the Unknown World

Despite decades of searching, we have yet to directly detect dark matter particles. But this is not a failure. The result of ‘finding nothing’ itself is valuable information telling us ’the suspect is not here,’ helping refine our investigation.

# Latest Frontline News

The recent LUX-ZEPLIN (LZ) experiment in the U.S. searched for WIMPs with unprecedented sensitivity but found no signals. This means that if WIMPs exist, they must interact with us far less than previously thought and hide in deeper realms.

As the possibility of WIMPs narrows, scientific interest naturally shifts to other suspects like axions. Research teams worldwide, including Korea’s Institute for Basic Science (IBS), are continuing the challenge to find axions with new technologies.

The Never-Ending Great Question

We have confirmed strong and consistent evidence that the invisible entity called dark matter designed the universe. Its existence is certain, but its identity remains shrouded in deep fog.

The quest to uncover dark matter’s identity is more than discovering a new particle. It is humanity’s grand intellectual exploration to understand where we came from, how this universe was made, and what the fundamental laws of nature are.

From detectors deep underground to telescopes navigating the cosmos, the day a decisive signal arrives will mark humanity’s completion of the true blueprint of the universe. Until then, the universe’s 95% remains a vast unknown waiting for us. And because of that mystery, our exploration continues.