What is dark matter?

There are a lot of things scientists don’t know about dark matter: Can we catch it in a detector? Can we make it in a lab? What kinds of particles is it made of? Is it made of more than one kind of particle? Is it even made of particles at all?

In short, dark matter is still pretty mysterious. The term is really just the name scientists gave to an ingredient that seems to be missing from our understanding of the universe. 

But there are some things scientists can definitively say about the stuff.

Natalia Toro is a theoretical physicist at the US Department of Energy’s SLAC National Accelerator Laboratory and a member of the Light Dark Matter Experiment (LDMX) and the Beam Dump Experiment (BDX) dark matter search. She gave a talk at the 2019 meeting of the American Physical Society’s Division of Particles and Fields about the short list of things we do know about dark matter. 

What Is Dark Matter?

Illustration by Sandbox Studio, Chicago with Ana Kova

Dark matter formed very early on in the universe’s history. The evidence of this is apparent in the cosmic microwave background, or CMB—the ethereal layer of radiation left over from the universe’s searingly hot first moments.  

The fact that so much dark matter still seems to be around some 13.7 billion years later tells us right away that it has a lifetime of at least 1017 seconds (or about 3 billion years), Toro says. 

But there is another, more obvious clue that the lifetime of dark matter is much longer than that: We don’t see any evidence of dark matter decay. 

The heaviest particles in the Standard Model of particle physics break down, releasing their energy in the form of lighter particles. Dark matter doesn’t seem to do that, Toro says. “Whatever dark matter is made of, it lasts a really long time.” 

This property isn’t unheard of—electrons, protons and neutrinos all have extremely long lifespans—but it would be unusual, especially if dark matter turns out to be heavier than those light, stable particles.

“One possibility is that there’s some kind of charge in nature, and dark matter is the lightest thing that carries that charge,” Toro says.

In particle physics, charge must be conserved—meaning it cannot be created or destroyed. Take the decay of a muon, a heavier version of an electron.

A muon often decays into a pair of neutrinos, which carry no charge, and an electron, which shares the muon's negative charge.

 So even though the muon has fallen apart into three other particles, its electromagnetic charge is conserved overall in the results of the decay.

The electron is the lightest particle with a negative electromagnetic charge. Since there’s nothing with a smaller mass for it to decay into, it remains stable. 

But the electromagnetic charge is not the only type of charge. Protons, for example, are the lightest particle to carry a charge called the baryon number, which is related to the fact that they’re made of particles called quarks (but not anti-quarks). Quarks and gluons have what physicists call color charge, which seems to be conserved in particle interactions.

It could be that dark matter particles are the most stable particles with a new kind of charge. 

What Is Dark Matter?

Illustration by Sandbox Studio, Chicago with Ana Kova

Dark matter’s apparent stability seems to have been key to another of its qualities: its ability to influence the evolution of the universe. Astrophysicists think that most galaxies would probably not have formed as they did without the help of dark matter. 

In the 1930s Swiss astrophysicist Fritz Zwicky noted that something seemed to be causing galaxies in the Coma Cluster to behave as if they were 400 times heavier than they would if they contained only luminous material. That discrepancy has today been calculated to be smaller, but it still exists. Zwicky coined the term “dark matter” to describe whatever could be giving the galaxies their extra mass.

In the 1970s Vera Rubin, an astronomer at the Carnegie Institution in Washington, used spectrographic evidence to determine that spiral galaxies such as our own also seemed to be acting more massive than they appeared. They were rotating far more quickly than expected, something that could happen if they were, for example, sitting in invisible halos of dark matter.

Scientists have seen another effect of dark matter on luminous material. Clusters of dark matter act as cosmic potholes on the path that light travels through the cosmos, bending and distorting it in a process called “gravitational lensing.” Astronomers can map the distribution of otherwise invisible dark matter by studying this lensing.

Dark Matter | COSMOS

Most of the mass-energy, about 95%, in the universe is ‘dark’. By dark we mean that it does not emit any form of electromagnetic radiation. The existence of Dark Matter is inferred indirectly by its gravitational effect.

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Observations of the motions of stars and gas in galaxies, cluster galaxy radial velocities, hot gas properties of clusters, and gravitational lensing of distant, background galaxies by foreground galaxy clusters all suggest large amounts of Dark Matter exist.

For example the observed radial velocities of cluster galaxies suggest dynamically-based cluster masses that are factors of 10 or more higher than that deduced by adding up the observed cluster mass (stars, gas, dust) content.

Dark Matter makes up 23% of the total mass-energy density of the universe. The dominant contributor is Dark Energy, and a small amount is due to atoms or baryonic matter.

What Is Dark Matter?

Schematic representation of the total mass-energy density in the universe.Credit: Swinburne

Comparisons of galaxy cluster and supercluster structure with N-body computational simulations also suggests the need for some sort of Dark Matter. Dark Matter is also needed for gravity to amplify the observed small fluctuations in the Cosmic Microwave Background to form the large-scale structure that we observe today.

What is dark matter?

What Is Dark Matter?

Since the 1930s, astrophysicists have been trying to explain why the visible material in galaxies can’t account for how galaxies are shaped, or how they behave. They believe a form of dark or invisible matter pervades our universe, but they still don’t know what this dark matter might be. Image via ScienceAlert.

Dark matter is a mysterious substance thought to compose perhaps about 27% of the makeup of the universe. What is it? It’s a bit easier to say what it isn’t.

It isn’t ordinary atoms – the building blocks of our own bodies and all we see around us – because atoms make up only somewhere around 5% of the universe, according to a cosmological model called the Lambda Cold Dark Matter Model (aka the Lambda-CDM model, or sometimes just the Standard Model).

Dark matter isn’t the same thing as dark energy, which makes up some 68% of the universe, according to the Standard Model.

Dark matter is invisible; it doesn’t emit, reflect or absorb light or any type of electromagnetic radiation such as X-rays or radio waves. Thus, dark matter is undetectable directly, as all of our observations of the universe, apart from the detection of gravitational waves, involve capturing electromagnetic radiation in our telescopes.

Yet dark matter does interact with ordinary matter. It exhibits measurable gravitational effects on large structures in the universe such as galaxies and galaxy clusters. Because of this, astronomers are able to make maps of the distribution of dark matter in the universe, even though they cannot see it directly.

They do this by measuring the effect dark matter has on ordinary matter, through gravity.

A long-lost type of dark matter may resolve the biggest disagreement in physics

One of the deepest mysteries in physics, known as the Hubble tension, could be explained by a long-since vanished form of dark matter. 

The Hubble tension, as Live Science has previously reported, refers to a growing contradiction in physics: The universe is expanding, but different measurements produce different results for precisely how fast that is happening.

Physicists explain the expansion rate with a number, known as the Hubble constant (H0). H0 describes an engine of sorts that’s driving things apart over vast distances across the universe.

According to Hubble’s Law (where the constant originated), the farther away something is from us, the faster it's moving.

And there are two main ways of calculating H0. You can study the stars and galaxies we can see, and directly measure how fast they're moving away. Or you can study the cosmic microwave background (CMB), an afterglow of the Big Bang that fills the entire universe, and encodes key information about its expansion. 

Related: The 11 Biggest Unanswered Questions About Dark Matter

As the tools for performing each of these measurements have gotten more precise, however, it's become clear that CMB measurement and direct measurements of our local universe produce incompatible answers.

What is dark matter?

When astronomers observe the movements of stars and galaxies, they run into a problem: What they see doesn't quite accord with what they would expect to see, given how much matter is visible and how much gravity it exerts. The swirling stars and galaxies seem to be under the sway of more matter than is observed. As a result, researchers believe there must be a large quantity of unseen matter along with the ordinary visible stuff.

This as-yet-undetected form of matter is invisible because it doesn’t interact with visible light or other forms of radiation. And yet evidence suggests that the universe contains more of this “dark matter” than the ordinary matter — protons, neutrons, and electrons — that we’re all familiar with. The nature of this dark matter is one of the biggest unsolved problems in all of physics.

Dark matter shouldn't be confused with “dark energy,” which is just as mysterious. Dark energy is a strange theoretical form of energy that scientists think is pushing every galaxy away from every other galaxy, causing the universe to expand at an accelerating rate.

Although the idea of dark matter dates back to the 19th century, it was a Caltech astrophysicist named Fritz Zwicky who in the 1930s developed the idea in its modern form.

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What is Dark Matter?

Roughly 80% of the mass of the universe is made up of material that scientists cannot directly observe. Known as dark matter, this bizarre ingredient does not emit light or energy. So why do scientists think it dominates?

Since at least the 1920s, astronomers have hypothesized that the universe contains more matter than seen by the naked eye. Support for dark matter has grown since then, and although no solid direct evidence of dark matter has been detected, there have been strong possibilities in recent years.

“Motions of the stars tell you how much matter there is,” Pieter van Dokkum, a researcher at Yale University, said in a statement. “They don't care what form the matter is, they just tell you that it's there.” Van Dokkum led a team that identified the galaxy Dragonfly 44, which is composed almost entirely of dark matter. [Image Gallery: Dark Matter Across the Universe]

The familiar material of the universe, known as baryonic matter, is composed of protons, neutrons and electrons. Dark matter may be made of baryonic or non-baryonic matter. To hold the elements of the universe together, dark matter must make up approximately 80% percent of the universe. The missing matter could simply be more challenging to detect, made up of regular, baryonic matter. 

Potential candidates include dim brown dwarfs, white dwarfs and neutron stars. Supermassive black holes could also be part of the difference. But these hard-to-spot objects would have to play a more dominant role than scientists have observed to make up the missing mass, while other elements suggest that dark matter is more exotic. 

Most scientists think that dark matter is composed of non-baryonic matter.

The lead candidate, WIMPS (weakly interacting massive particles), have ten to a hundred times the mass of a proton, but their weak interactions with “normal” matter make them difficult to detect.

Neutralinos, massive hypothetical particles heavier and slower than neutrinos, are the foremost candidate, though they have yet to be spotted. 

Sterile neutrinos are another candidate. Neutrinos are particles that don't make up regular matter.

A river of neutrinos streams from the sun, but because they rarely interact with normal matter, they pass through the Earth and its inhabitants.

There are three known types of neutrinos; a fourth, the sterile neutrino, is proposed as a dark matter candidate. The sterile neutrino would only interact with regular matter through gravity.

Dark Energy, Dark Matter | Science Mission Directorate

In the early 1990s, one thing was fairly certain about the expansion of the universe.

It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on.

Granted, the slowing had not been observed, but, theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together.

Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.

Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a “cosmological constant.” Maybe there was some strange kind of energy-fluid that filled space.

Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name.

It is called dark energy.

What Is Dark Energy?

More is unknown than is known. We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the universe is dark energy.

Dark matter makes up about 27%. The rest – everything on Earth, everything ever observed with all of our instruments, all normal matter – adds up to less than 5% of the universe.

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Come to think of it, maybe it shouldn't be called “normal” matter at all, since it is such a small fraction of the universe.

Universe Dark Energy-1 Expanding Universe

This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.

5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.

Credit: NASA/STSci/Ann Feild

One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood.

The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: “empty space” can possess its own energy.

Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster.

Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe. 

Dark Matter Core Defies Explanation

This image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. The result could present a challenge to basic theories of dark matter.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, “empty space” is actually full of temporary (“virtual”) particles that continually form and then disappear.

But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong – wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it.

It's hard to get an answer that bad. So the mystery continues.

Dark Matter and Dark Energy

The visible universeincluding Earth, the sun, other stars, and galaxiesis made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up less than 5 percent of the mass of the universe.

The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent).

Scientists have not yet observed dark matter directly. It doesn't interact with baryonic matter and it's completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments. But scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.

For instance, according to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a galaxy's visible matter is concentrated.

But observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk.

This puzzling result makes sense if one assumes that the boundary stars are feeling the gravitational effects of an unseen massdark matterin a halo around the galaxy.

Dark matter could also explain certain optical illusions that astronomers see in the deep universe. For example, pictures of galaxies that include strange rings and arcs of light could be explained if the light from even more distant galaxies is being distorted and magnified by massive, invisible clouds of dark matter in the foreground-a phenomenon known as gravitational lensing.

Scientists have a few ideas for what dark matter might be. One leading hypothesis is that dark matter consists of exotic particles that don't interact with normal matter or light but that still exert a gravitational pull. Several scientific groups, including one at CERN's Large Hadron Collider, are currently working to generate dark matter particles for study in the lab.

Other scientists think the effects of dark matter could be explained by fundamentally modifying our theories of gravity. According to such ideas, there are multiple forms of gravity, and the large-scale gravity governing galaxies differs from the gravity to which we are accustomed.

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