Tales from the dark side What do we really know about dark matter? Some type of mysterious, invisible mass holds the universe together. Here’s how scientists are searching for it. by Liz Kruesi Massive galaxy clusters like Abell 2218 have pro- vided scientists with evidence for the existence of dark matter. By analyzing the arcs — warped images of a background galaxy — surrounding the cluster center, astronomers determined the cluster must have a lot more mass than meets the eye. NASA/Andrew Fruchter/ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI) he universe doesn’t abide by “what it doesn’t emit X-rays or absorb infrared plotted the velocity versus the distance to Astronomers study the sizes and makes up most of their baryonic mass. Unlike anything we’ve seen you see is what you get.” In fact, radiation). This mysterious stuff is there- create a “rotation curve.” They expected shapes of those arcs to determine a clus- Ordinary matter interacts through elec- For many years astronomers thought dark the stuff we see in space — stars, fore invisible, yet astronomers learned it the velocities to reach a maximum and ter’s mass. By comparing that calculated tromagnetic forces. Thus, as matter col- matter could consist of dead stars, black gas, and dust — accounts for only exists because dark matter interacts with then decrease farther from the center — mass to the mass that comes from only lides it loses energy as radiation (in the holes, and other known objects that emit 10 percent of the universe’s mass. ordinary matter through gravity. but the data showed otherwise. The luminous objects (the galaxies), astrono- form of X-rays, in this case). The hot gas little or no light. They used gravitational TThis visible stuff is ordinary matter, and velocities reach a maximum and then mers can determine how much dark slows during the collision. microlensing to look for these objects. it’s made up of protons, neutrons, and Searching in the dark plateau. With velocities so high at the matter is in a cluster. Astronomers use gravitational lensing This technique is similar to gravitational electrons. Scientists call ordinary matter Swiss astrophysicist Fritz Zwicky first outer edge of galaxies, the stars should Other evidence has turned up in colli- to indirectly map the dark matter distri- lensing except the foreground object is “baryonic matter” because protons and proposed dark matter’s existence in 1933. fling out of their orbits. But they don’t. sions of galaxy clusters, namely the Bullet bution. It turns out that dark matter also much less massive. Instead of light bend- neutrons are subatomic particles called While studying the Coma cluster of gal- Some sort of mass scientists can’t detect cluster. This object is actually the after- passed through the collision unaffected. ing around the object, the body’s gravity baryons. The other 90 percent of the mass axies, he found that the galaxies’ collec- must be holding these outer stars in orbit. math of two galaxy clusters that collided. So images of the Bullet cluster show magnifies the light from behind it. While is “dark matter,” and it likely surrounds tive gravity alone was much too small to A very massive object — such as a gal- Astronomers used a multidetection direct evidence of dark matter. astronomers found some of these MAssive almost every galaxy in the universe. hold the cluster together. axy cluster — can act as a gravitational approach to look at the galaxies, gas, and The evidence is piling up — with 3-D Compact Halo Objects (MACHOs), there Dark matter doesn’t emit, absorb, or The next round of evidence came in lens. Some images of regions around gal- dark matter. When the clusters collided, dark matter maps and other detections. weren’t enough to account for all of the reflect any type of light (so, for example, the 1970s. Astronomers charted the axy clusters show numerous arcs. Those the galaxies’ stars passed through mostly Yet mapping the distribution is one thing; universe’s missing mass. velocities of stars at various distances are background galaxies distorted and unaffected because a lot of space exists knowing the characteristics of this myste- So if dark matter isn’t composed of Liz Kruesi is an associate editor of Astronomy. from the center of a spiral galaxy and magnified by the cluster’s gravity. between the stars. The clusters’ hot gas rious stuff is another story. normal objects, then it likely consists of 28 Astronomy • November© 2014 Kalmbach 09 Publishing Co. This material may not be reproduced in any www.Astronomy.com 29 form without permission from the publisher. www.Astronomy.com that each ordinary particle (such as an Simulating structure electron or quark) has a massive “super- partner” (a selectron or squark) that remains undetected. The leading dark matter candidate is a class of particles that supersymmetry predicts. These particles have mass and interact through the weak nuclear force, but they don’t interact through the elec- tromagnetic force. Because these weakly interacting massive particles (WIMPs) interact via the weak force, they can col- lide with normal atomic nuclei and bounce off them without emitting light or absorbing radiation. The lightest Klypin (NMSU) Chicago)/Anatoly (U. Kravtsov Andrey WIMP — called the neutralino — is also The universe’s structure seems to evolve as smaller clumps bunch to form larger structures. Astronomers simulate structure evolution with cold dark matter the most popular dark matter possibility. and create models that resemble today’s universe. This simulation shows dark matter distribution, where brighter areas represent more dense regions. Another common CDM candidate is the axion. The axion is also a hypotheti- matter in a detector. A WIMP can collide cal particle, but it arises from a theory with an atomic nucleus and move, or different from supersymmetry. This par- “scatter,” the nucleus. ticle is not a “matter particle” but instead Another method is to indirectly detect a force carrier, similar to the photon dark matter. A WIMP’s antiparticle is NASA/ESA/CXC/M. Bradac (University of California, Santa Barbara)/S. Allen (Stanford University) (Stanford Allen Santa Barbara)/S. of California, (University Bradac NASA/ESA/CXC/M. (which “carries” the electromagnetic itself, so if two WIMPs interact, they Two massive galaxy clusters collided to form what’s known as the Bullet cluster of galaxies. Ordi- force). It’s much lighter than a WIMP — annihilate each other and produce a nary matter — the hot gas, shown in pink — collided, lost energy, and slowed. The clusters’ dark at least 1 billion times less massive — so shower of secondary particles. Astro- matter (shown in blue) interacted little and passed through the ordinary matter. the universe would need a whole lot physicists can observe many of these sec- more axions than WIMPs to make up all ondary particles — such as electrons, non-baryonic particles — meaning it’s with dark matter,” says Juan Collar of the the invisible matter. positrons (the electron’s antiparticle), not made up of the same stuff as ordinary University of Chicago. “However, these One would expect that with so many gamma rays, and neutrinos. matter (protons and neutrons). Astrono- hypothetical particles turn out to have all CDM particles, WIMPs or axions would Scientists’ methods to locate axions mers split non-baryonic dark matter into of the properties (mass, abundance, life- be easy to find. But because they don’t are “totally different than the direct and two categories: hot and cold. These titles time, probability, and mode of interac- interact through the electromagnetic indirect detection methods used to look have nothing to do with temperature. tion) required to be the dark matter, or at force, detecting them pushes scientists’ for WIMPs,” says Dan Hooper of Fermi Hot means that early in the universe least a fraction of it.” experimental limits. National Accelerator Laboratory in Bata- these particles traveled extremely fast — For decades, physicists have worked to via, Illinois. When an axion traverses a almost at the speed of light. Cold means explain how the four fundamental physi- How to hunt CDM detector that has a magnetic field, it will that early in the universe the particles cal forces fit together. (These forces are The method used to detect dark matter convert into a photon. of Nottingham, U.K.)/ (University Gray Vancouver)/M. of British Columbia, (University NASA/ESA/C. Heymans U.K.)/K. Meisenheimer (Max University, (Oxford Wolf collaboration/C. (Innsbruck)/the M. Barden STAGES collaboration COMBO-17 Heidelberg)/the Astronomy, Institute for Planck traveled more slowly. gravitation, electromagnetism, weak depends on what type of dark matter Instead of trying to detect CDM par- Supercluster Abell 901/902 contains hundreds of galaxies. Astronomers analyzed the distortion of How does particle speed relate to dark nuclear, and strong nuclear.) In the past (WIMP or axion) scientists pursue. Sci- ticles, some scientists aim to create the some 60,000 background galaxies’ shapes to determine the supercluster’s distribution of dark mat- ter. They then combined a visible-light image of the supercluster with the dark matter distribution matter’s composition? Slower particles 30 or so years, they’ve arrived at super- entists looking for WIMPs try to directly particles — WIMPs and axions — in the map (shown as a magenta haze). will bunch up into small structures ear- symmetry theory. This model predicts observe the interaction with ordinary laboratory. To do this, they have to gen- lier in the universe. Those small struc- erate extremely high energies, similar to tures will eventually collide and merge to A rotation curve those shortly after the Big Bang. Only it moves through this dark matter haze.