What if most of the universe is made of something we can’t see or touch? That’s dark matter. Dark matter makes up about 25% of the total mass in the universe but we can’t observe dark matter directly or indirectly. That’s five times more common than the normal matter we see, interact with, and detect.

We see dark matter’s effects—gravity pulls on galaxies and bends light from stars. Among many questions we have about dark matter, one of the big ones is what is dark matter made of? How can it take up so much mass in the universe without interacting with anything except for gravity? There are all kinds of theories about what makes up dark matter. Some think it could be tiny particles that don’t interact with light. In contrast, others suggest larger, more unusual forms of matter. Yet, no experiment has detected these mysterious particles or confirmed what dark matter is.

Check this out, too: Dark Matter and Black Holes: What’s the Connection?

In this post, I’ll go through the most popular ideas about what dark matter is made of. There are many possible answers, from strange particles like WIMPs and axions to theoretical black holes from the early universe.

What is Dark Matter: Introduction

Dark matter is the mysterious stuff that makes up most of the universe’s mass. Imagine the things we can see in space, like stars, planets, and galaxies—they only account for about 15% of all the mass. The other 85%? That’s dark matter and dark energy (25% dark matter, 60-70% dark energy). We can’t see, feel, or touch it because it doesn’t interact with light or energy in the usual way. It’s invisible but still there, affecting everything with its gravity.

Think of dark matter like wind: we can’t see the wind itself, but we know it’s there because we can see trees sway and leaves move. In the same way, scientists know this matter exists because they see its pull on galaxies and stars. Galaxies, for instance, spin faster than they should if only visible matter held them together. Something extra—dark matter—must add this additional force.

While scientists still don’t know what it is, they believe it’s made of particles different from the ones that make up the stars and planets. This is precisely why we wonder what is dark matter made of because it has to be something we need to understand.

The Composition of Dark Matter: Current Theories

The most popular theory is that dark matter could be WIMPs—short for Weakly Interacting Massive Particles. These hypothetical particles would barely interact with anything, making them almost impossible to detect. If WIMPs exist, they’d add mass without giving off light. This matches what we know about dark matter.

Another theory is axions. They are tiny particles that are so light and low-energy that they hardly interact with other matter. Scientists think axions might be floating through space, adding invisible mass on a large scale. Like WIMPs, axions would be hard to spot directly, but some labs are looking for tiny hints of their presence.

There’s also an idea that dark matter could be primordial black holes, ancient black holes formed right after the Big Bang. These wouldn’t shine like stars but would add mass, pulling on nearby objects with gravity. If enough of them are out there, they might add up to explain dark matter. This last one is one of the most plausible theories for dark matter.

What Is Dark Matter Made Of?

I briefly mentioned some theories about what dark matter is made of and its composition. They are still theories, and we don’t have any evidence to say one is true. Dark matter particles are still a mystery. I want to elaborate on some of the most plausible theories here. They are still very brief, so you can write a whole book on each theory if you want to go deeper into these. The book “Dark Matter and the Dinosaurs” by Lisa Randall explains these much more in detail – and much better than me.

Weakly Interacting Massive Particles (WIMPs)

WIMPs are a top choice for dark matter. They can weigh between 1 and 1,000 times as much as a proton. Scientists use special detectors like XENONnT, with 3.2 tons of liquid xenon, to find WIMPs. But so far, we haven’t detected any WIMPs. They’re called “weakly interacting” because, aside from gravity, they hardly affect regular matter at all. This makes them nearly impossible to detect directly because they don’t emit or reflect light. Think of WIMPs as tiny, elusive “ghost” particles that drift through space without affecting the things we can see. Since they don’t interact with light, it’s hard to detect them.

Axions: A Lightweight Candidate

We consider axions as another option for dark matter. They are proposed as tiny, ultra-light particles that interact very little with normal matter. Unlike WIMPs, axions have almost no mass and don’t carry any electric charge, which makes them even harder to detect. However, in large numbers, axions could add up to form the invisible mass that dark matter is believed to be. Scientists look for hints of their presence through experiments using strong magnetic fields. They hope to see axions transform into photons (particles of light) under special conditions.

Primordial Black Holes

Primordial black holes (PBHs) are another intriguing theory for dark matter. Unlike regular black holes, which form when massive stars collapse, PBHs could have formed right after the Big Bang in extremely high-density areas. If enough of these ancient black holes exist, their combined mass could account for the hidden matter scientists see affecting galaxies. PBHs wouldn’t emit light, so they’re invisible like other dark matter candidates, but their gravity would still affect surrounding objects. Researchers look for PBHs by watching for “microlensing” events, where a PBH might pass in front of a distant star and temporarily make it appear brighter.

The Dark Sector: A New Frontier

The Dark Sector theory is a new idea. It says there are particles that interact with each other a lot but not with regular matter. Unlike ordinary matter, which we understand fairly well, the dark sector could include particles that don’t interact with light or regular forces, making them invisible and challenging to study. This sector might include particles like WIMPs or axions but also opens the door for other undiscovered forms of matter and energy.

Scientists suggest that the dark sector could have its own forces, like a “dark electromagnetic force,” where particles interact only within the dark sector, separate from the forces we experience. Exploring the dark sector could help a lot. It won’t just explain dark matter, it could also explain the way the universe expands and holds together.

Detecting Dark Matter

There are two ways to detect dark matter – direct and indirect. Direct is the best way possible if we want to have solid evidence about dark matter. However, this is extremely hard because dark matter doesn’t interact with anything. Indirect dark matter, on the other hand, can be easier. It’s still a challenge, and we still haven’t found any indirect proof, but with time, this could yield faster and better results.

Direct Detection Experiments

Direct dark matter detection experiments aim to observe dark matter particles by detecting their rare interactions with ordinary matter. We use ultra-sensitive equipment to conduct these experiments. The aim is to search for faint signals indicating the presence of dark matter particles, like WIMPs. Here are a few primary methods:

• Cryogenic Detectors: Cool materials to very low temperatures to detect tiny changes in energy when dark matter particles collide with atoms.
• Noble Gas Detectors: Use gases like xenon or argon to detect flashes of light or ionization caused by particle interactions.
• Scintillation Detectors: Observe light emitted by certain crystals when a dark matter particle collides with an atom inside the crystal.

These experiments are set up deep underground to reduce interference from cosmic rays. It gives scientists a better chance to detect the elusive signals of dark matter.

Indirect Detection Experiments

Indirect detection methods look for signals that dark matter particles might produce when they decay or collide with each other. Scientists observe regions in space where dark matter is dense, like the centers of galaxies, and search for unusual signals that could hint at dark matter interactions. They use both ground-based and space-based observatories for this.

• Gamma-Ray Observations: Use telescopes to detect gamma rays that might be emitted when dark matter particles annihilate.
• Neutrino Detectors: Search for neutrinos produced by dark matter interactions near massive objects like the Sun.
• Cosmic-Ray Detectors: Detect cosmic rays, especially positrons or antiprotons, that might result from dark matter collisions.

The XENONnT experiment has set a record for detecting WIMP dark matter. It can spot particles with a very small cross-section. XENONnT is a potential new experiment that the scientists will launch to study dark matter further.

Dark Matter’s Role in the Universe

Dark matter makes up over 25% of the universe. Without dark matter, galaxies would spin apart, and the universe would collapse because there is nothing binding them together. They couldn’t stay bound by only the pull of visible matter. This unseen mass acts as a cosmic “glue,” creating the framework on which galaxies and galaxy clusters form.

In the early universe, dark matter was one of the core matters that played a role in shaping the cosmic web. The cosmic web is a vast network of galaxies and dark matter filaments. These filaments connect galaxy clusters and form the large-scale structure of the universe. Dark matter’s gravitational influence on normal matter helped stars and galaxies form in the first place.

Conclusion

Dark matter is still one of the biggest mysteries in physics and astronomy, and each theory we’ve explored sheds a bit more light on what is dark matter made of. We don’t have an answer!

Looking at the elusive WIMPs and axions to the ancient primordial black holes, I believe we are coming closer to understanding what is dark matter made of and learning the composition of dark matter. The dark sector adds another layer to this puzzle. There might be a whole range of particles and forces that we can’t yet detect, but they shape the cosmos in unseen ways.

Through direct and indirect detection experiments, researchers work to capture signals that could confirm one or more of these dark matter candidates. Observing how dark matter affects galaxies and the large-scale structure of the universe also gives us insight into its role as the unseen “glue” that binds galaxies, supports cosmic structures, and impacts the universe’s evolution.

Future experiments like SuperCDMS and hidden-sector detectors are promising, though. These efforts follow discoveries like the 42 supernovas found by the Hubble Space Telescope. This discovery was a breakthrough and won the Nobel Prize in Physics in 2011.

FAQ

What is dark matter?

Dark matter is an invisible substance that makes up about 25% of all matter in the universe. It interacts very weakly with ordinary matter but exerts gravitational attraction.

What evidence do we have for dark matter?

We have evidence from galaxy rotation, galaxy cluster dynamics, and cosmic structure formation. These observations show dark matter’s presence, but we didn’t detect dark matter directly. So we can’t really call them evidence, but rather hypothesis.

What are the leading theories about the composition of dark matter?

Scientists have proposed several candidates. These include Weakly Interacting Massive Particles (WIMPs), axions, primordial black holes, and particles from the “Dark Sector” or “Hidden Sector.”

What are WIMPs?

WIMPs (Weakly Interacting Massive Particles) are hypothesized to have 1 to 1,000 times more mass than a proton. They are considered a leading candidate for dark matter.

What are axions?

Axions are much lighter particles. They are proposed to solve theoretical issues in particle physics, with about one trillionth the mass of an electron.

How are scientists searching for dark matter?

Scientists use direct detection experiments with large, sensitive detectors underground. They also use indirect detection through cosmic rays and gamma rays. Additionally, they try to create dark matter using particle accelerators.

What is the Hidden Sector in dark matter research?

The Hidden Sector, also known as the Dark Sector, is a theory. It proposes that dark matter particles exist in their own universe. They have independent dynamics and interact weakly with normal matter.