What is dark matter made of

Abell Pandora's Cluster Revealed. One of the most complicated and dramatic collisions between galaxy clusters ever seen is captured in this new composite image of Abell The blue shows a map of the total mass concentration mostly dark matter. Researchers were surprised when they uncovered galaxy NGC DF2 which is missing most, if not all, of its dark matter.

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Where is the Universe's Missing Matter? The current racecourse weaves from the grandest scales of the universe to the tiniest, from galaxies to subatomic particles. The odds-on favorite, called a WIMP for weakly interacting massive particle , has been a no-show despite intensive search efforts.

Meanwhile, a once highly touted competitor called the massive compact halo object, or MACHO — cheekily named in opposition to the WIMP — has fallen out of contention, its very existence debunked. Some newer long shots, meanwhile, are poised to give the dark matter thoroughbreds a run for their money. Particle masses are measured in units called gigaelectron volts, or GeV. When it comes to dark matter, physicists have placed their biggest bets, in terms of research dollars, on WIMPs.

These entities surged to the fore in the mids, connecting the largest, cosmic scale of physics with the smallest, the standard model of particle physics. Developed over many decades, the standard model is a stunning scientific success.

But the model also has yawning gaps, including not being able to describe the fourth force, gravity, and failing to explain dark matter at all. A refinement of the standard model called supersymmetry smooths over many of its flaws. It fills the gaps by proposing new, heavier partner particles for all known particles. Cosmologists had already been kicking around the idea of WIMPs without knowing what they might be, and suddenly they had a match.

Emphasis on the should. Despite multiple big-budget experiments in and , WIMPs have disappointed. WIMPs also have failed to appear in other detection methods. Theories suggest the particles may occasionally destroy each other or decay, resulting in showers of gamma rays, but searches have found no convincing evidence.

And many physicists expected that the Large Hadron Collider — the most powerful particle accelerator ever built — would produce heavy, novel particles, including WIMPs. But a decade of operations with no heavy partners to show for it has instead made some physicists question the whole notion of supersymmetry. After a humble start, the axion is now surging in the race.

The standard model is perfectly cool with this, but it bothered researchers, so they came up with a way to explain that unusual rigidness. As a side effect, the explanation also suggested the universe may be full of new hypothetical particles called axions. And, as it happens, axions also fit the bill for dark matter. Although the individual particles have a ridiculously low mass, the universe-forming Big Bang could have churned out axions in dizzying abundance — enough, in fact, to constitute all the dark matter in the cosmos.

Thanks to an upgrade announced in , ADMX became the first device with the sensitivity necessary for nabbing those hyper-aloof axions. Essentially, a magnet inside cranks out a powerful magnetic field that, according to theory, should convert any nearby axions into your standard radio waves.

This article was originally published with the title "What Is Dark Matter? Lisa Randall is Frank B. Baird, Jr. She serves on Scientific American 's board of advisers.

Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Read more from this special report: The Biggest Questions in Science. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism. Knowledge awaits. See Subscription Options Already a subscriber? Dark matter's effects are most dominant, on average, in the smallest galaxies of all.

This one's a little bit counterintuitive, but has been observationally validated practically everywhere we look. Under the laws of gravitation, all forms of matter are treated equally. But the other forces, like nuclear and electromagnetic forces, only affect normal matter. When a large burst of star formation takes place in a galaxy, all of that radiation simply passes through the dark matter, but it can collide with and be absorbed by the normal matter.

This means that if your galaxy is low enough in mass overall, that normal matter can be expelled by intense episodes of star formation. The smaller and lower-in-mass your galaxy is, the greater the amount of normal matter that will be expelled, while all the dark matter will remain.

In the most striking examples of all, dwarf galaxies Segue 1 and Segue 3, both satellites of the Milky Way, contain only a few hundred stars, but some , solar masses of material overall. The dark matter-to-normal matter ratio is approximately to-1, as opposed to 5-to-1 in most large-scale structures.

Four colliding galaxy clusters, showing the separation between X-rays pink and gravitation blue , On large scales, cold dark matter is necessary, and no alternative or substitute will do.

However, mapping out the X-ray light pink is not necessarily a very good indication of the dark matter distribution blue. Dark matter causes gravitational effects in places where normal matter isn't located. This is some of the strongest evidence of all that dark matter cannot simply be normal matter that's dark.

When two galaxy groups or clusters collide, the intergalactic gas and plasma collides and heats up, emitting X-rays shown in pink. This represents the overwhelming majority of the normal matter, far more than what's found in stars and the individual galaxies themselves. But the signal from the mass, inferred from gravitational lensing, illustrates that the majority of the mass is located where the blue contours are shown. This can only be true, given the wide variety of colliding clusters where this has been demonstrated, if some new form of mass obeys different collisional laws than normal matter does.

The inescapable conclusion is that some new form of matter — dark matter — must make up the majority of the Universe's mass. However, just because there are things we know about dark matter doesn't mean that we know it all. In fact, here are five major things we don't know about it. The quest for particle dark matter has led us to look for WIMPs that may recoil with atomic nuclei. We don't know what particles are responsible for dark matter, or if it's even a particle at all. We know that dark matter exists, that it doesn't interact significantly with itself, normal matter, or radiation, and that it's cold.

But we don't know what properties it actually has. Dark matter could be:. But all of our efforts to directly detect a candidate particle or field for dark matter have come up empty. We see its astrophysical effects indirectly, and that's indisputable, but on particle-sized scales, we have no idea what's going on. The presence, type, and properties of dark matter clumps can influence the particular variations The fact that we now have detailed spectroscopic data on eight of these systems allows meaningful information to be extracted about the nature of dark matter.

We don't know whether the "dark sector" is simple or rich.