Science & Discovery
Discovering the Dark Universe
Most of the universe is undetectable, and yet astronomers have learned an incredible amount about this invisible and mysterious part of the cosmos in the past five decades.
By Liz Kruesi
NASA/ESA/M.J. Jee and H. Ford (Johns Hopkins University)
Humans understand the world around us via observation. By interpreting the glowing dots in the night sky, early observers began to understand the greater cosmos. Eventually they learned those pricks of light are stars, some a few light years away and others far more distant and making up galaxies. Observers later could see other types of light, like penetrating X-rays and other high-energy radiation, which are released during explosive cosmic events. And they could detect the microwave hum from a spinning compact star.
Such observations over the centuries have taught us about the universe and our place within it. But these observations can also reveal what we don’t know — because we cannot see all.
That’s how scientists in the past few decades learned that everything they can see, all the material they understand and detect, makes up only about 5 percent of the universe. Of the remaining 95 percent of the cosmos, about one-third is an invisible matter, and the remaining is something even more mysterious. To uncover this majority of the universe, researchers have developed new experiments, detection methods, and theoretical explanations.
Only 5 percent of the universe is ordinary matter. The remaining 95 percent is invisible dark matter and mysterious dark energy.
[NASA/CXC/K. Divona]
In the 1970s, multiple groups of astronomers used both optical and radio telescopes to measure the velocities of stars and bright nebulas in the Andromeda Galaxy (M31). The findings suggested the existence of an unseen type of matter.
[Vera Rubin and Janice Dunlap]
Early Observations
The first evidence of invisible dark matter came in the 1930s, when astronomer Fritz Zwicky looked at the motions of galaxies within a cluster of galaxies. He analyzed the motions of several member galaxies in the Coma cluster and calculated the mass required to keep the galaxies gravitationally bound together. It was substantially more than his calculation of the galaxy cluster’s mass based on its luminous galaxies. That meant, “it would have the surprising result that dark matter is present in the universe in far greater density than visible matter,” he wrote in his 1933 paper in Helvetica Physica Acta (this quote has been translated from the original German paper).
Fast forward to the crucial results of the 1970s, which finally convinced scientists of this missing matter. Just two years before the Astronomical Society of the Pacific launched Mercury, two astronomers — Vera Rubin and W. Kent Ford — published their first results from studying the motions of stars and glowing gaseous regions in the nearest large galaxy to us, the Andromeda Galaxy. They were using these objects’ spread-out light, their spectra, to measure how fast those stars and emission regions were moving around the center of their galaxy. Their orbiting motions should follow gravity, the same type of motions that govern how the eight planets orbit the Sun (closer in means moving faster). But when the astronomers looked at the speeds of the objects located in the middle of Andromeda’s disk, things looked odd. Those stars and gaseous regions were moving faster than expected. Some type of mass the astronomers couldn’t see seemed to gravitationally hold those nebulas to the galaxy. This result echoed Zwicky’s findings of galaxies in clusters.
A few years later, in 1975, astronomers Mort Roberts and Robert Whitehurst published their findings of the Andromeda Galaxy. Their radio observations extended how far out from the galaxy’s center they could see. The bright regions’ velocities stayed constant.
Throughout that decade, more researchers looked at additional galaxies, and they found the same story. By the early 1980s, most astronomers realized the majority of the universe’s material was dark and undetectable directly.
Researchers develop large computer simulations to track cosmic evolution and better understand how dark matter works. This slice shows dark matter’s distribution at multiple zoom levels (the smallest clumps in the most zoomed-in image are the mass of Earth).
[MPA]
The DEAP-3600 detector is currently operating some 1.2 miles (2 kilometers) below Sudbury, Ontario, Canada. When particles (like background signals and other types of cosmic particles — or dark matter) traverse the liquid-argon-filled experiment, their interactions with this material could initiate an ultraviolet light signal.
[DEAP-3600 Experiment]
A cosmic glue
Physics, and its laws, describes how everything in the universe operates under four fundamental forces: the gravitational force (astronomers and us on Earth are very familiar with this one, but it’s actually the weakest force), the electromagnetic force (optical light and other forms of radiation), the strong nuclear force (it holds atomic nuclei together), and the weak force (it’s responsible for radioactive decay). The latter three forces work in conjunction with the elementary particles that physicists know to comprise the so-called “Standard Model;” the gravitational force doesn’t quite fit within that same framework, and summarizing that discrepancy is worth an entire other article. Scientists’ understanding of everything they can detect and interact with is based upon the Standard Model and the general theory of relativity, the latter of which tells us how gravity works. Astronomers have looked for direct signals of dark matter for decades, but they’ve seen signs of this invisible material interacting with the ordinary cosmos through only the gravitational force — everything dominated by the Standard Model.
The gravitational force affects anything with mass, whether it emits or absorbs light or it’s invisible. This force governs how cosmic structure grew and evolved. Scientists know dark matter influenced cosmic structure, because it is the dominant form of matter. It is a sort of cosmic glue, says Queen’s University researcher Gopolang Mohlabeng. “It’s an unobserved type of matter that binds our galaxies together.”
In the early universe, higher densities of matter, which was mostly dark and not what we know as ordinary, formed what astronomers call “gravitational wells” that attract anything with mass. Matter would fall in, would become trapped, in these wells. As more of that ordinary matter falls in and it reaches higher densities, because it interacts via the electromagnetic force, it heats up and eventually releases radiation. Stars are born. Galaxies are born. Cosmic structure is born.
“If you think about it,” says Mohlabeng “without dark matter actually trapping normal matter to form our Milky Way Galaxy, the Solar System would not have formed, the Earth would not have formed, and we actually wouldn’t be here.”
After decades trying to learn more about dark matter, astronomers unfortunately are confident of only two things: It exists, and it interacts via gravity. “The hardest part about searching for dark matter is that it can be a lot of things,” adds NASA astrophysicist Regina Caputo.
The Cosmic Microwave Background carries information about the first several hundred thousand years of cosmic history. From that information, scientists know how structures evolved, how dark matter influences regular matter, and how much dark energy exists.
[ESA – C. Carreau]
Detecting the invisible
What researchers really want is to directly detect this mysterious matter — that means watching it bump into special underground detectors, seeing its interaction signatures in the nearby universe, or perhaps even creating dark matter in Earth’s most-powerful physics experiments. For decades, scientists have designed, built, and run complex, and some less-complex, detectors. They’ve created advanced computer simulations to guide them. But it’s complicated, because there are so many theories of what dark matter could be and each idea requires a specific technique to find it.
One of the leading theories for what dark matter could be is a class of particles known as Weakly Interacting Massive Particles (WIMPs). Theorists in the 1980s first presented such a hypothesis, which fit within the framework of the Standard Model. It was one that particle physicists, astronomers, and cosmologists thought could explain dark matter. And so, experimentalists built detectors and placed them underground, observers used telescopes to look toward galaxies, particle physicists tried to create it in massive colliders — all to find these WIMPs.
But those searches and experiments have all came up empty. And it doesn’t mean WIMPs aren’t still an option, it just means they can’t be all of dark matter.
“You always look at the easiest thing first, right, and we did that, and we didn’t see it,” says Caputo. She’s an observational astrophysicist who’s been looking for gamma-ray signals corresponding to WIMP interactions in nearby galaxies. “And so now we have to just keep diving deeper to try to figure out what [dark matter] is.”
Astronomers are now looking to more complex and varied models. Caputo provides an analogy of a soccer field. “[Imagine] you’re looking at individual blades of grass to try to figure out … what makes up the soccer field,” she says. “Is the grass the same on the other side of the field as it is on my side of the field?”
And not just that, but there is nothing to say all the dark matter is the same. Ordinary matter, after all, isn’t only one type of particle. Instead, there’s a dozen elementary particles and a handful of force carrier particles, interacting with one another in multiple ways. Dark matter could be just as complex — if not more. This invisible matter is some 80–85 percent of the total mass in the universe.
“The fact that it doesn’t interact with light, it doesn’t interact with everyday things that we see and touch,” says Mohlabeng, “makes it very difficult for us to actually know where to begin to search.” As a dark matter phenomenologist, he looks at the data astrophysicists have and then conceives new models that could explain dark matter. Observational and experimental researchers then develop ways to test those models.
And it’s a field that is becoming ever more creative, as dark matter researchers investigated other possible candidates for the universe’s missing matter, some 26 percent of the total universe. That amount added to normal matter makes up 31 percent. The remaining roughly 69 percent is something even more mysterious, more elusive, and far less understood than dark matter.
The Fermi Gamma-ray Space Telescope has spied an excess of gamma rays at the center of our galaxy. This radiation could be from dark matter particle interactions. [NASA/DOE/Fermi LAT Collaboration and T. Linden (U. of Chicago)]
The collisions between clusters of galaxies display evidence that dark matter exists. The more-dispersed hot gas, the magenta pink X-ray glow in this image, was slowed by friction during the collision; while the blue in this image traces the mass distribution of dark matter (calculated from warped images of background galaxies).
[X-ray (NASA/ CXC/Stanford/S. Allen); Optical/Lensing (NASA/STScI/U.C. Santa Barbara/M. Bradac)]
Serendipitous findings
Astronomers categorize objects all the time to understand similarities and the physics behind them. In the 1980s, astronomers discovered exploding stars called “supernovas” come in two main classes. Within one of those broader categories is a subclass called “type 1a,” and these, it turns out, are crucial tools for understanding the universe.
Type 1a supernovas all have similar intrinsic brightnesses whenever or wherever they explode, and those similarities mean astronomers can use them to measure distances across the universe. By comparing how a Type 1a supernova’s light changed over time, scientists could figure out how much energy it was emitting and thus how far away from Earth it was.
In the following decade, two teams detected dozens of these supernovas at different distances from us, where a farther distance translates to further back in time. Scientists have known since the 1930s that the universe is actually expanding, and the supernova researchers used the brightness-distance relationship to study the rate the universe was expanding. Saul Perlmutter led one team, and Brian Schmidt and Adam Reiss led the other. They expected to measure how much the expansion had slowed due to the gravitational pull of all the matter in the universe, but instead, as the teams announced in 1998, they found the expansion has actually sped up. The supernovas were fainter and thus farther away than expected, and that would happen only if something was pulling the universe apart. Enter “dark energy,” a mysterious something that seems to be counteracting gravity.
“If these results are confirmed, it will require a major change in our picture for the universe,” stated astrophysicist Robert Kirshner in a 1999 Proceedings of the National Academy of Sciences paper. “We will be forced to add another constituent to our best model for the universe, a form of vacuum energy that drives the expansion.”
One could argue the brightness calculations weren’t correct, or maybe there’s a whole lot of dust and other material making the supernova blasts appear fainter and redder. But scientists over the decades since have investigated these options and ruled them out.
They’ve now measured hundreds of Type 1a supernovas, confirming these results over and over again, and they’ve also employed other probes to measure cosmic expansion.
The vast majority of our universe is dark, like this clever jelly bean display.
[NASA/Chandra]
Fossil signals
One of these observational techniques uses a pervasive fossil radiation, the residual glow from early in the universe’s history. When the cosmos first came into existence 13.8 billion years ago, it was incredibly dense and extraordinarily hot. All of the current universe’s matter and radiation was crammed together at that time, and then the cosmos expanded rapidly. At the beginning, the matter and radiation moved together until about 400,000 years later, when the light was no longer linked with matter, and it could travel freely. All that once-high-energy radiation has rode along with cosmic expansion since, which has pulled that blazing light to cool microwaves.
Now, this cool radiation is everywhere in the universe, known as the Cosmic Microwave Background (CMB). Astronomers have studied this leftover radiation with many detectors over the past several decades, and they can read much of the information it carries. That includes the seeds of today’s largest structures — cosmic scaffolding of galaxies and clusters of galaxies — and evidence that the universe is dominated by a dark energy component. In fact, it’s the CMB that reveals 69 percent of the cosmos is dark energy.
The background temperature of the universe, shown in different colors, differs by just fractions of a degree. This cosmic background radiation eventually evolved into today’s cosmic structure.
[ESA and the Planck Collaboration]
The universe’s expansion is accelerating, and if it continues, we could end up with a “Big Rip.” This diagram shows that future scenario and a few additional possibilities.
[NASA/CXC/M. Weiss]
That’s not the only additional tool astronomers have used. By mapping the large structures across the sky, like clusters of galaxies across time, scientists can tell how dark energy has counteracted gravity.
While astrophysicists have confirmed this mysterious energy exists, they have essentially no idea what it is. One theory is it’s the energy of the vacuum of space itself. “In quantum physics, the vacuum should have energy, thanks to Heisenberg’s uncertainty principle,” says University of Queensland astrophysicist Tamara Davis. “You can never actually be sure that nothing is there.”
Particles appear for very short amounts of time and then disappear, and these particles would give energy to the vacuum. “Vacuum energy should have exactly the property we expect dark energy to have: In particular, it would have negative pressure,” she says. But there’s a big mismatch in the amount of vacuum energy needed to explain dark energy and the amount of it actually expected to exist. That mismatch is many factors of 10.
“We talk about dark energy as though it’s a thing,” adds Davis. “It could also be that our theory of gravity is wrong and that what dark energy is is actually a correction to our theory of gravity.”
The European Space Agency’s Euclid spacecraft, shown in this illustration, is now in space and studying the dark universe.
[ESA]
What's next?
Planned telescope surveys will constrain more details of dark energy, further measuring how it affects the universe. And while that will help scientists understand this puzzle, it won’t reveal what dark energy actually is. That’s because observation and experiment are only one part of the equation. “We can keep measuring its properties in ever more detail, using many different methods, to understand how it behaves,” says Davis. “However, to understand what it is, we need new theoretical breakthroughs — which is the hard part.”
It’s a similar story for dark matter: theoretical innovations will lead the observers and experimentalists in their quest to understand the universe's invisible material.
“When we first started thinking about particles and what makes up the fundamental building blocks of the universe, it took a hundred years at least to figure that out,” says Caputo. Now, scientists are trying to understand 95 percent of the universe, and they have far fewer tools to use. They cannot see dark matter and they cannot detect dark energy. They can only use how these mysterious entities affect the remaining 5 percent of the cosmos. ✰
(Originally published August 2021)
LIZ KRUESI is the Editor of Mercury. She has told the stories of the universe since 2005, and is especially interested in the research that reveals the dark side of the cosmos.
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