Enlarge / A compact dwarf galaxy, which may have features that are difficult to explain with standard models of dark matter. (credit: ESA/Hubble & NASA )

It is impossible for a telescope to image and far from being completely understood, yet dark matter is everywhere.

The deepest mysteries about dark matter relate to its nature and behavior. The prevailing idea regarding dark matter is the cold dark matter theory (CDM), which posits that dark matter is made up of low-velocity particles that do not interact with each other. This thinking has been debated—and it is up for debate again. Led by astrophysicist Hai-Bo Yu, a team of researchers from the University of California Riverside have come up with an alternative idea that explains two extremes where cold dark matter doesn't work well.

Galaxies and galaxy clusters are thought to be surrounded by halos of dark matter . At one end of the controversy are galactic dark matter halos that are too dense to be consistent with CDM, and at the other are galactic dark matter halos too diffuse for CDM to make sense of. Yu and his colleagues instead suggest that some dark force (sorry Star Wars fans—not the Force) causes dark matter particles to smash into each other. This is Self-Interacting Dark Matter, or SIDM.

Enlarge / One of the first galaxies that Euclid observed is nicknamed the "Hidden Galaxy." This galaxy, also known as IC 342 or Caldwell 5, is difficult to observe because it lies behind the busy disk of our Milky Way. (credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi )

The European Space Agency released the first five science images from the Euclid space telescope Tuesday, showing how the wide-angle observatory will survey familiar cosmic wonders like galaxies and stars to study the unseen dark energy and dark matter that dominate the Universe.

Stationed nearly a million miles (1.5 million kilometers) from Earth, Euclid will scan one-third of the sky over the next six years, collecting an estimated 1 million images of billions of galaxies. Scientists have developed sophisticated algorithms to analyze the data coming down from Euclid to measure the distances and shapes of each of these galaxies.

From that, scientists can infer how the influence of dark matter pulls on the galaxies, forming clusters and causing them to spin faster. Dark energy is the mysterious force that is driving the accelerated expansion of the Universe.

Enlarge / SpaceX's Falcon 9 rocket soars through the sky over Cape Canaveral with Europe's Euclid space telescope. (credit: Stephen Clark/Ars Technica)

A European Space Agency telescope launched Saturday on top of a SpaceX Falcon 9 rocket from Florida to begin a $1.5 billion mission seeking to answer fundamental questions about the unseen forces driving the expansion of the Universe. The Euclid telescope, named for the ancient Greek mathematician, will observe billions of galaxies during its six-year survey of the sky, measuring their shapes and positions going back 10 billion years, more than 70 percent of cosmic history.

Led by the European Space Agency, the Euclid mission has the ambitious goal of helping astronomers and cosmologists learn about the properties and influence of dark matter and dark energy , which are thought to make up about 95 percent of the Universe. The rest of the cosmos is made of regular atoms and molecules that we can see and touch.

Stumbling in the dark

“To highlight the challenge we face, I would like to give the analogy: It’s very difficult to find a black cat in a dark room, especially if there’s no cat,” said Henk Hoekstra, a professor and cosmologist at Leiden Observatory in the Netherlands. “That’s a little bit of the situation we find ourselves in because we have these observations … But we lack a good theory. So far, nobody has come up with a good explanation for dark matter or dark energy.”

Enlarge / Gravitational lensing has revealed a previously unknown supernova explosion more than 4 billion light-years away. (credit: Joel Johansson, Stockholm University)

Astronomers have detected a previously unknown supernova explosion more than 4 billion light-years away using a rare phenomenon called "gravitational lensing," which serves as a kind of cosmic magnifying glass. They described their discovery and its potential implications in a new paper published in the journal Nature Astronomy. Co-author Ariel Goobar, director of the Oskar Klein Center at Stockholm University, described the find as "a significant step forward in our quest to understand the fundamental forces shaping our universe."

Gravitational lensing is a direct consequence of the general theory of relativity: mass bends and warps spacetime, and light must follow that curvature. The phenomenon can form rare effects like an " Einstein ring " or an " Einstein cross ." Essentially, the distortion in space-time caused by a massive object (like a galaxy) acts as a lens to magnify an object in the background. Since these aren't perfect optical-quality lenses, there are often some distortions and unevenness. This causes the light from the background object to take different paths to Earth, and thus a single object can appear in several different locations distributed around the lens. At cosmological scales, those paths can also require light to travel very different distances to get to Earth.

Gravitational lensing helps astronomers spot celestial objects that might otherwise be too faint or far away to see, like a distant supernova, which can lead to other interesting questions. For example, last year , astronomers analyzed a Hubble image from 2010, where the image happened to also capture a supernova. Because of gravitational lensing, the single event showed up at three different locations within Hubble's field of view. Thanks to the quirks of how this lensing works, and because light travels at a finite speed, all three of the locations captured different times after the star's explosion, allowing researchers to piece together the time course following the supernova, even though it had been observed over a decade earlier.

Enlarge / The red arcs at the right of center are background galaxies distorted by gravitational lensing. The number, location, and degree of distortion of these images depends on the distribution of dark matter in the foreground. (credit: ESA/Hubble & NASA, A. Newman, M. Akhshik, K. Whitaker )

Decades after it became clear that the visible Universe is built on a framework of dark matter, we still don't know what dark matter actually is. On large scales, a variety of evidence points toward what are called WIMPs: weakly interacting massive particles. But there are a variety of details that are difficult to explain using WIMPs, and decades of searching for the particles have turned up nothing, leaving people open to the idea that something other than a WIMP comprises dark matter.

One of the many candidates is something called an axion , a force-carrying particle that was proposed to solve a problem in an unrelated area of physics. They're much lighter than WIMPs but have other properties that are consistent with dark matter, which has sustained low-level interest in them. Now, a new paper argues that there are features in a gravitational lens (largely the product of dark matter) that are best explained by axion-like properties.

Particle or wave?

So, what's an axion? On the simplest level, it's an extremely light particle with no spin that acts as a force carrier. They were originally proposed to ensure that quantum chromodynamics, which describes the behavior of the strong force that holds protons and neutrons together, doesn't break the conservation of charge parity. Enough work was done to make sure axions were compatible with other theoretical frameworks, and a few searches were done to try to detect them. But axions have mostly languished as one of a number of potential solutions to a problem that we haven't figured out how to resolve.

Produced and directed by Corey Eisenstein. Click here for transcript . (video link)

Ars' Edge of Knowledge series looks at important aspects of our Universe that we still understand poorly, like dark matter and the origin of life. This week, our host Paul Sutter bravely ventures into the area we probably understand the least: dark energy. Dark energy accounts for about 75 percent of the stuff in the Universe, but we still don't have even the slightest idea what it is and are a bit stumped as to how to even go about finding out.

Paul goes into how we accidentally discovered that the Universe's expansion is accelerating when astronomers looked for an indication that the expansion was slowing down. Dark energy is simply the term we're using for the big unknown here: What's driving that acceleration?

Enlarge / The dark matter-poor galaxies are so diffuse that you can see right through them. (credit: NASA, ESA, and P. van Dokkum )

The Universe's first galaxies are thought to have formed at sites where a lot of dark matter coalesced, providing the gravitational pull to draw in enough regular matter to create stars. And, to date, it's impossible to explain the behavior of almost all the galaxies we've observed without concluding that they have a significant dark matter component.

Almost, but not all. Recently, a handful of galaxies have been identified that are dim and diffuse, and appear to have relatively little dark matter. For a while, these galaxies couldn't be explained, raising questions about whether the observations had provided an accurate picture of their composition. However, researchers recently identified one way the galaxies could form : A small galaxy could be swallowed by a larger one that keeps the dark matter and spits out the stars.

Now, a second option has been proposed, based on the behavior of dark matter in a galaxy cluster. This model may explain a series of objects found near the dark matter-poor galaxies. And it may suggest that galaxy-like objects could be formed without an underlying dark matter component.