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 1. The Universe


WHAT IS DARK MATTER?

References
By Nola Taylor Tillman
Contributions from
Tereza Pultarova
last updated August 01, 2023

Dark matter is the mysterious stuff that fills the universe but no one has ever
seen.

 * 
 * 
 * 
 * 
 * 
 * 
 * 


Roughly 80% of the mass of the universe is made up of dark matter but what is
it? (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)
Jump to:
 * What is dark matter?
 * Dark matter Q&A with an expert
 * How do we know it exists?
 * Where does it come from?
 * How do we study dark matter?
 * Additional resources
 * Bibliography

Dark matter makes up over 80% of all matter in the universe, but scientists have
never seen it.  



We only assume it exists because, without it, the behavior of stars, planets and
galaxies simply wouldn't make sense. 



Here is what we know about it, or rather, what we think we know. 




Related: If dark matter is 'invisible,' how do we know it exists?


WHAT IS DARK MATTER AND WHY IS IT INVISIBLE?

Dark matter is completely invisible. It emits no light or energy and thus cannot
be detected by conventional sensors and detectors. The key to its elusive nature
must lie in its composition, scientists think.



Visible matter, also called baryonic matter, consists of baryons — an
overarching name for subatomic particles such as protons, neutrons and
electrons. Scientists only speculate what dark matter is made of. It could be
composed of baryons but it could also be non-baryonic, that means consisting of
different types of particles. 

Most scientists think that dark matter is composed of non-baryonic matter. The
lead candidate, WIMPS (weakly interacting massive particles), are believed to
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 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.

"One of the outstanding questions is whether there is a pattern to the fractions
that go into each neutrino species," Tyce DeYoung, an associate professor of
physics and astronomy at Michigan State University and a collaborator on the
IceCube neutrino observatory in Antarctica, told Space.com.

The smaller neutral axion and the uncharged photinos — both theoretical
particles — are also potential placeholders for dark matter.

There is also such a thing as antimatter, which is not the same as dark matter.
Antimatter consists of particles that are essentially the same as visible matter
particles but with opposite electrical charges. These particles are called
antiprotons and positrons (or antielectrons). When antiparticles meet particles,
an explosion ensues that leads to the two types of matter canceling each other
out. Because we live in a universe made of matter, it is obvious that there is
not that much antimatter around, otherwise, there would be nothing left. Unlike
dark matter, physicists can actually manufacture anti-matter in their
laboratories. 

Related: Image Gallery: Dark matter across the universe




DARK MATTER Q&A WITH AN EXPERT

We asked Glenn Starkman, a Distinguished University Professor and co-Chair of
Physics and Professor of Astronomy at Case Western Reserve University, a few
frequently asked questions about gravity. 

Glenn Starkman
Social Links Navigation
Distinguished University Professor and co-Chair of Physics and Professor of
Astronomy

Distinguished University Professor and co-Chair of Physics and Professor of
Astronomy at Case Western Reserve University, Director of the Institute for the
Science of Origins, Director of the Center for Education and Research in
Cosmology and Astrophysics. 


DOES DARK MATTER EXIST?

I wish I knew! What we do know is that if we look at a typical galaxy, take
account of all the matter that we see (stars, gas, dust) and use Newton's Laws
of Gravity and motion (or, more correctly, Einstein's General Relativity), to
try to describe the motions of that material, then we get the wrong answer. The
objects in galaxies (nearly all of them) are moving too fast. There should not
be enough gravity to keep them from flying out of the galaxy that their in. The
same thing is true about galaxies moving around in clusters.

There are two possible explanations:

1. There is more stuff (matter) that we don't see with our telescopes. We call
this dark matter.

2. Newton's laws and even GR are wrong on the scale of galaxies and everything
bigger. This idea is usually called modified gravity (because we need to modify
GR) or Modified Newtonian Dynamics (MOND).

Mostly, cosmologists believe that the answer is that the behavior of galaxies is
explained by dark matter. Why? Partly. because it has been very hard to write
down a successful theory of MOND or modified gravity. And partly because it
turned out that when we turned our microwave telescopes to look at cosmic
background radiation (CMB), the light from the early universe, it turned out
that, according to GR, the same amount and type of dark matter was also required
to explain the behavior of the sound waves that traveled in the universe when it
was less than 500,000 years old, and whose imprints we are able to see. Modified
gravity struggles to provide a unified explanation across all these systems —
galaxies, clusters of galaxies, the universe.

But we don't yet know what the dark matter is made of.


DOES DARK MATTER HAVE MASS?

If dark matter exists, it must have mass. Massless dark matter would not behave
in ways that solve the problems that dark matter addresses. 


WHAT DOES DARK MATTER DO?

The two things we know for sure about dark matter (assuming it exists), is that
it exerts gravity (has mass) and that it moves slowly (compared to the speed of
light).  


HOW DO YOU LOOK FOR DARK MATTER?

Since we don't know what dark matter is, the answer is: for every possible
candidate for the dark matter there is a different strategy to search for it.
People build giant detectors deep underground (to get away from all the other
particles streaming through the environment around us) and look for signals of
the dark matter hitting their detector after passing through the Earth overhead.

I happen to be looking for a form of dark matter that is quite massive — between
about 100g and many tonnes — and would have more easily visible effects. But
because it is massive, it is very rare. (We know how much total mass of dark
matter there should be, so if the individual dark matter particles are heavy,
there are fewer of them.) For example, if it hit a rock, it would melt the rock
along its path as it zoomed through the rock. We can look for the scars of those
passages, for example, in granite counter tops.


WHY DO WE THINK DARK MATTER EXISTS?

But if we cannot see dark matter, how do we know it exists? The answer is
gravity, the force exerted by objects made of matter that is proportional to
their mass. Since the 1920s, astronomers have hypothesized that the universe
must contain more matter than we can see because the gravitational forces that
seem to be at play in the universe simply appear stronger than the visible
matter alone would account for.

"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." 

Astronomers examining spiral galaxies in the 1970s expected to see material in
the center moving faster than at the outer edges. Instead, they found the stars
in both locations traveled at the same velocity, indicating the galaxies
contained more mass than could be seen. 

Studies of gas within elliptical galaxies also indicated a need for more mass
than found in visible objects. Clusters of galaxies would fly apart if the only
mass they contained was the mass visible to conventional astronomical
measurements.

Different galaxies seem to contain different amounts of dark matter. In 2016, a
team led by Van Dokkum found a galaxy called Dragonfly 44, which seems to be
composed almost entirely of dark matter. On the other hand, since 2018
astronomers have found several galaxies that seem to lack dark matter
altogether. 

The force of gravity doesn't only affect the orbits of stars in galaxies but
also the trajectory of light. Famous physicist Albert Einstein showed in the
early 20th century that massive objects in the universe bend and distort light
due to the force of their gravity. The phenomenon is called gravitational
lensing. By studying how light is distorted by galaxy clusters, astronomers have
been able to create a map of dark matter in the universe.

A vast majority of the astronomical community today accepts that dark matter
exists. 

"Several astronomical measurements have corroborated the existence of dark
matter, leading to a world-wide effort to observe directly dark matter particle
interactions with ordinary matter in extremely sensitive detectors, which would
confirm its existence and shed light on its properties," the Gran Sasso National
Laboratory in Italy (LNGS) said in a statement. "However, these interactions are
so feeble that they have escaped direct detection up to this point, forcing
scientists to build detectors that are more and more sensitive."

Despite all the evidence pointing towards the existence of dark matter, there is
also the possibility that no such thing exists after all and that the laws of
gravity describing the motion of objects within the solar system require
revision.



Dark matter appears to be spread across the cosmos in a network-like pattern,
with galaxy clusters forming at the nodes where fibers intersect. (Image credit:
WGBH)


WHERE DOES DARK MATTER COME FROM?

Dark matter appears to be spread across the cosmos in a net-like pattern, with
galaxy clusters forming at the nodes where fibers intersect. By verifying that
gravity acts the same both inside and outside our solar system, researchers
provide additional evidence for the existence of dark matter. (Things are even
more complicated as in addition to dark matter there also appears to be dark
energy, an invisible force responsible for the expansion of the universe that
acts against gravity.)

But where does dark matter come from? The obvious answer is that we don't know.
But there are a few theories. A study published in December 2021 in The
Astrophysical Journal argues that dark matter might be concentrated in black
holes, the powerful gates to nothing that due to the extreme force of their
gravity devour everything in their vicinity. As such, dark matter would have
been created in the Big Bang together with all other constituting elements of
the universe as we see it today. 

Stellar remnants such as white dwarfs and neutron stars are also thought to
contain high amounts of dark matter, and so are the so-called brown dwarfs,
failed stars that didn't accumulate enough material to kick-start nuclear fusion
in their cores. 



A mysterious glow coming from the center of the Milky Way might be caused by
annihilating dark matter. (Image credit: Mattia Di Mauro (ESO/Fermi-Lat))


HOW DO SCIENTISTS STUDY DARK MATTER?

Since we can't see dark matter, can we actually study it? There are two
approaches to learning more about this mysterious stuff. Astronomers study the
distribution of dark matter in the universe by looking at the clustering of
material and the motion of objects in the universe. Particle physicists, on the
other hand, are on a quest to detect the fundamental particles making up dark
matter. 

An experiment mounted on the International Space Station called the Alpha
Magnetic Spectrometer (AMS) detects antimatter in cosmic rays. Since 2011, it
has been hit by more than 100 billion cosmic rays, providing fascinating
insights into the composition of particles traversing the universe. 

"We have measured an excess of positrons [the antimatter counterpart to an
electron], and this excess can come from dark matter," Samuel Ting, AMS lead
scientist and a Nobel laureate with the Massachusetts Institute of Technology,
told Space.com. "But at this moment, we still need more data to make sure it is
from dark matter and not from some strange astrophysics sources. That will
require us to run a few more years."

Back on Earth, beneath a mountain in Italy, the LNGS's XENON1Tis hunting for
signs of interactions after WIMPs collide with xenon atoms. 

"A new phase in the race to detect dark matter with ultra-low background massive
detectors on Earth has just begun with XENON1T," project spokesperson Elena
Aprile, a professor at Columbia University, said in a statement. "We are proud
to be at the forefront of the race with this amazing detector, the first of its
kind."

The Large Underground Xenon dark-matter experiment (LUX), seated in a gold mine
in South Dakota, has also been hunting for signs of WIMP and xenon interactions.
But so far, the instrument hasn't revealed the mysterious matter.

"Though a positive signal would have been welcome, nature was not so kind!" Cham
Ghag, a physicist at University College London and collaborator on LUX, said in
a statement. "Nonetheless, a null result is significant as it changes the
landscape of the field by constraining models for what dark matter could be
beyond anything that existed previously."

Related content:

— Hunting for dark matter — inside the Earth

— Dark matter hasn't killed anybody yet — and that tells us something

— Dark matter can form tiny, cold 'clumps.' Scientists have found the smallest
ones yet.

The IceCube Neutrino Observatory, an experiment buried under the frozen surface
of Antarctica, is hunting for the hypothetical sterile neutrinos. Sterile
neutrinos only interact with regular matter through gravity, making it a strong
candidate for dark matter.

Experiments aiming to detect elusive dark matter particles are also conducted in
the powerful particle colliders at the European Organization for Nuclear
Research (CERN) in Switzerland.

Several telescopes orbiting Earth are hunting for the effects of dark matter.
The European Space Agency's Planck spacecraft, retired in 2013, spent four years
in the Lagrangian Point 2 (a point in the orbit around the sun, where a
spacecraft maintains a stable position with respect to Earth), mapping the
distribution of the cosmic microwave background, a relic from the Big Bang, in
the universe. Irregularities in the distribution of this microwave background
revealed clues about the distribution of dark matter. 

In 2014, NASA's Fermi Gamma-ray Space Telescope made maps of the heart of our
galaxy, the Milky Way, in gamma-ray light, revealing an excess of gamma-ray
emissions extending from its core.

"The signal we found cannot be explained by currently proposed alternatives and
is in close agreement with the predictions of very simple dark matter models,"
lead author Dan Hooper, an astrophysicist at Fermilab in Illinois, told
Space.com.

The excess can be explained by annihilations of dark matter particles with a
mass between 31 and 40 billion electron volts, researchers said. The result by
itself isn't enough to be considered a smoking gun for dark matter. Additional
data from other observing projects or direct-detection experiments would be
required to validate the interpretation.

The James Webb Space Telescope, launched after 30 years of development on Dec.
25, 2021, is also expected to contribute to the hunt for the elusive substance.
With its infrared eyes able to see to the beginning of time, the telescope of
the century won't be able to see dark matter directly, but through observing the
evolution of galaxies since the earliest stages of the universe, it is expected
to provide insights that have not been possible before.  

ESA's Euclid mission launched on July 1, 2023, and is currently on the hunt for
dark matter and dark energy. The mission aims to map the geometry of matter in
the universe, specifically the distribution of galaxies to learn more about the
elusive dark matter. 


ADDITIONAL RESOURCES

You can read more about dark matter on the website of the U.S. Fermi National
Accelerator Laboratory (Fermilab), which runs high-energy experiments in cutting
edge particle colliders with the goal of discovering particles that would fill
the gaps in our understanding of the universe. The European Organization for
Nuclear Research (CERN), the largest particle physics laboratory in the world,
is also on a quest to find missing dark matter particles. NASA discusses the
difference between dark matter and dark energy in this article.


BIBLIOGRAPHY

NASA, Dark Energy, Dark Matter

https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

Clegg, B. Dark Matter and Dark Energy: The Hidden 95% of the Universe, Icon
Books, August, 2019

CERN, Dark Matter

https://home.cern/science/physics/dark-matter

This article was update on Jan 28, 2022 by Space.com Senior Writer Tereza
Pultarova.

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Nola Taylor Tillman
Social Links Navigation
Contributing Writer

Nola Taylor Tillman is a contributing writer for Space.com. She loves all things
space and astronomy-related, and enjoys the opportunity to learn more. She has a
Bachelor’s degree in English and Astrophysics from Agnes Scott college and
served as an intern at Sky & Telescope magazine. In her free time, she
homeschools her four children. Follow her on Twitter at @NolaTRedd

With contributions from
 * Tereza PultarovaSenior Writer

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