It sounds like science fiction to say there’southward invisible, undetectable stuff all around us, and it doesn’t help that it has the spooky proper name of dark matter. But at that place’due south enough of evidence that this fabric is very real. And then what exactly is night matter? How practice we know it’s at that place? And how are scientists looking for it?

Everything we see around u.s. – from plants to planets, stones to stars, people to the Perseus galaxy cluster – is fabricated of affair. But all of this only accounts for about 15 percent of the total matter in the universe. The overwhelming majority, that remaining 85 percent, is unaccounted for – and we telephone call it dark thing.

This name isn’t describing what this strange stuff looks like – it earns that title considering it doesn’t blot, reflect or refract light, making it finer invisible. And there’southward nothing that can explicate it in the Standard Model of particle physics, which remains our best theory on the universe.

There’south a huge worldwide attempt to try to uncover what night matter actually is, merely the natural question information technology raises is: if we tin can’t see it, feel it, hear, scent or taste it, how do we know it exists at all?

The answer is gravity.

How practise we know nighttime thing is there?

Night matter is believed to pervade the universe – so why oasis’t we plant it yet? And how do nosotros even know it’s there?

X-ray: NASA / CXC/ U. Victoria/ A. Mahdavi et al. Optical/Lensing: CFHT/ U. Victoria/ A. Mahdavi et al.

Anything that has mass has a gravitational pull, and the more mass something has, the stronger this force becomes. Just astronomers consistently see that big-scale objects like galaxies and clusters behave like they take much more than mass than what’s visible.

Swiss astrophysicist Fritz Zwicky was the outset to propose the idea of dark matter in 1933. He was studying a cluster of galaxies and establish a discrepancy: in that location didn’t seem to be anywhere near plenty mass to account for how fast those galaxies were moving.

“When he looked at all the visible contributions to the mass that was in the cluster, he found that there was a huge shortfall in terms of providing the gravity that was needed to go on these relatively fast-moving galaxies from just leaving the cluster,” Raymond Volkas, Professor of Theoretical Particle Physics at the University of Melbourne, tells New Atlas. “Why was the cluster holding together at all? And then he surmised that there had to be another component that was gravitating but was invisible to u.s.a.. And he called it dark thing.”

Zwicky’s discovery was simply the kickoff example of plainly missing mass. In the late 1970s, astronomers Vera Rubin and Kent Ford were observing our neighboring galaxy, Andromeda. The duo expected to meet objects on the fringes of the milky way orbiting more slowly than those closer to the heart, but that wasn’t the case: instead, relative speeds tended to flatten off, with objects on the fringes orbiting far faster than the visible mass should let.

Another strong piece of bear witness is gravitational lensing. Since light beams are distorted past gravitational fields, huge masses can bend light passing by from more distant objects and make those objects announced larger or brighter, like a cosmic magnifying glass. Other times information technology can indistinguishable the image of an object, or even “replay” events like supernovae. Again, this lensing often occurs more strongly than should be possible from the visible mass of the object in the middle.

Then nosotros know nighttime matter is there. Just it gets weirder – the universe as we know it couldn’t exist without dark matter.

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The Dark History of the Universe

A simulated view of the distribution of dark matter in our universe
A imitation view of the distribution of nighttime matter in our universe

Virgo Consortium

Just similar the regular stuff, dark matter is believed to have been created in the Large Bang – or equally one theory suggests, fifty-fifty before it, during a menstruum of cosmological inflation. Either manner, the construction we meet out in the cosmos today would be very unlike without dark affair.

In the early days of the universe, everything was relatively smooth. We can see this today in the catholic microwave background, which is radiation that was created some 400,000 years subsequently the Big Bang. No matter which management nosotros await in, this radiations looks exactly the same.

Merely nowadays, the universe is far from smooth – information technology’s quite clumpy. These clumps are what we see equally galaxies, clusters, superclusters, and other gigantic structures, and there’s always relatively empty space in between them. For example, right next door to the Milky Way is the “Local Void,” a region of unfathomable pettiness that spans hundreds of millions of light-years.

Then how did the universe evolve from super smooth to clumpy clusters? That’s dark matter’south influence at work.

Even in the shine early days of the universe, some regions had slightly more nighttime matter than others. This extra mass meant greater gravity, so these denser areas then attracted regular thing, which in turn attracted more than and more. Eventually the heat and pressure caused these pockets of thing to ignite as stars, kickstarting the formation of the planetary systems, galaxies, and clusters that we see today.

The fact that the universe is structured the mode information technology is is further evidence of dark matter. And then nosotros know it’s there. Merely what exactly is it? And how are scientists searching for it?

The hunt for night thing

The ABRACADABRA experiment detected no signals of axions with masses between 0.31 and 8.3 nanoelectronvolts
The Abracadabra experiment detected no signals of axions with masses between 0.31 and viii.3 nanoelectronvolts


Information technology’s not like shooting fish in a barrel to wait for something that’due south invisible and rarely interacts with regular thing. So, scientists start past theorizing what dark matter could
be, and and so design and conduct experiments to test each hypothesis. The problem is, nighttime matter could be nigh anything.

Night matter particles could exist amid the lightest in the universe, or they could take the mass of a dwarf planet, or anywhere in betwixt. Night matter could be “hot” or “cold,” which has aught to do with temperature just describes how fast it moves. It could be in excited states, or have lower energies.

“Famously there’s an embarrassing plethora of different hypothetical particles or sets of particles that nighttime matter could be,” Volkas tells us. “And the range of masses and other backdrop for these night matter candidates is enormous, so we simply don’t know.

“Theoreticians are very skillful at coming up with speculations most what the dark matter could exist, and most of them are very sensible speculations. So they could all be true in principle – simply they won’t all exist truthful at once. And so what nosotros need to practise are experiments and astronomical observations to try to narrow downwardly the possibilities and come at the truth.”

Can CERN create dark matter?

A 3D render of the Large Hadron Collider

A 3D render of the Large Hadron Collider

Unlike types of experiments are hunting for unlike theoretical night thing particles. Perhaps the most famous experiments are those existence conducted by CERN, at the Big Hadron Collider (LHC). At that place, scientists are looking for dark matter by trying to

In the LHC, protons are made to collide at extremely high energies, producing a shower of other particles. Sometimes those are exotic particles that scientists ordinarily wouldn’t have access to, and the hope is that night matter may be amid them.

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Again though, if dark affair was produced in i of these collisions, information technology would be incommunicable to directly detect – instead it would just float off out of the tunnel without interacting with the detector. Only that not-detection is exactly what scientists are looking for.

In physics, the laws of Conservation of Energy and Momentum land that in an isolated system, neither energy nor momentum can be created or destroyed. They may alter course, merely the corporeality will stay constant. And then the scientists can calculate how much energy and momentum went in before the proton collision, and measure how much in that location is subsequently. If there’southward anything missing, that suggests that something – like nighttime affair – escaped and carried away that free energy or momentum.

Although the LHC has performed quadrillions of these collisions over the years, so far no suspicious dark matter signal has been spotted. Only that helps to narrow downward the broad range of possibilities, so future searches can be more focused.

Maybe the answer volition finally come up after the LHC’southward High-Luminosity upgrade is completed in 2026.

Direct detection of dark matter

The XENON1T facility, on the left is the water tank containing the instrument itself, with a poster showing what's inside – on the right is the three-story service building
The XENON1T facility, on the left is the water tank containing the instrument itself, with a poster showing what’s inside – on the right is the three-story service building

The Xenon Collaboration

While the LHC is searching in one office of the spectrum of possibility, other experiments are trying to detect it in different ways. These studies are banking on the chance that dark matter may sometimes interact with regular thing through means other than gravity.

“The LHC is only sensitive to some kinds of night matter,” Volkas says. “In that location are other sensible dark matter candidates for which the LHC is the incorrect experiment. Some other style to wait for dark matter is through something called straight detection experiments. So the thought is you lot have a suitably large detector, you put information technology in a very serenity environs which is free from background influences that could mimic your dark thing signal, and then you but watch the detector and you wait for the nucleus of an atom to suddenly nothing off for no credible reason. The idea is that a night affair particle has come along, hitting the nucleus and caused it to zip off.”

This basic concept has been put into exercise in various experiments all around the earth. The detectors are ordinarily placed in deep underground chambers, away from interference like cosmic rays or electromagnetic signals. And they’re all searching for different hypothetical dark thing particles, using different substances as the detector.

Experiments like LUX and XENON1T used huge tanks of xenon to endeavour to detect a dark thing candidate known every bit a weakly-interacting massive particle (WIMP). The idea being that when these theoretical WIMPs bump into a xenon atom in the tank, they would give off a wink of calorie-free that instruments tin can find.

Another proposal would use superfluid helium instead. The logic is that helium has a much lighter atomic nucleus than xenon, so it should be more sensitive to a bump from dark matter. That means it could pick up nighttime matter particles that are x,000 times lighter than other experiments.

A variation on the idea is what’s been called a “snowball chamber.” This proposal uses a tank of pure water that’south supercooled to -twenty ° C (-4 °F). At those subzero temperatures, the slightest disturbance to the h2o molecules can trigger flash freezing. And then if information technology suddenly ices over for no credible reason, that could exist a dark matter betoken. The advantage at that place is that water is far cheaper and easier to come by than xenon or superfluid helium.

Other scientists are tackling the trouble in a completely different fashion. One proposed detector suggests using an array of a billion tiny pendulums, suspended in an extremely even so environs. If a night matter particle happens to whiz through the instrument, its gravitational influence should ready a row of these pendulums swinging.

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Missing in axion

An illustration of the axion radio design, with the axions (wavy lines) passing through
An illustration of the axion radio design, with the axions (wavy lines) passing through

Alexander Millar/Stockholm University

1 of the leading dark affair candidates is a hypothetical particle called an axion. If they exist, these would be electrically neutral, very light, and drift around everywhere in waves. But most importantly, they should have tiny but detectable interactions with electricity and magnetism – and that just might be how they reveal themselves.

The ABRACADABRA experiment is designed to look for the magnetic fingerprint of axions. The idea is that considering of how electromagnetic fields piece of work, at that place should be no magnetic field in the very center of a ring-shaped magnet. So if you prepare ane up and spotter the centre, an axion could brand itself known if a magnetic field spontaneously arises at that place.

In a similar idea, scientists at Stockholm University proposed a device they telephone call an “axion radio.” The detector likewise uses a powerful magnet, but at the center is a chamber filled with cold plasma that contains a forest of ultrathin wires. This time, any axions passing through would produce a modest electric field that would drive oscillations in the plasma.

The nEDM experiment is searching for axions in a unlike way. Here, neutrons are trapped and electrified, and so their spin is monitored. The high voltage should touch their charge per unit of spin at a certain frequency – and if that frequency is seen to vary over fourth dimension, it could exist a sign of axion interference.

In a similar experiment, physicists at Fermi Lab are hunting for axions using a supercooled, superconducting cavity containing a quantum flake (qubit) antenna. The idea there is that the magnetic field could convert passing night matter particles into photons in the cavity, where the antenna can discover them.

Astronomical observations

Dark matter particles, in the form of ultralight bosons, could sap angular momentum from black holes in the same way as jumping on and off a carousel does
Dark thing particles, in the grade of ultralight bosons, could sap angular momentum from black holes in the same way every bit jumping on and off a carousel does

Jose-Luis Olivares, MIT

But nosotros don’t have to just search for dark matter in our own lawn – the stuff pervades the universe, so perhaps nosotros should exist looking out into the cosmos for information technology.

Astronomers have suggested that a night affair candidate called a sterile neutrino could decay into photons and normal neutrinos, giving off 10-ray emissions in the process. Looking for these otherwise unexplained X-ray bursts in galaxies could be a smoking gun for dark matter.

Similarly, axions could be turned into visible photons in the farthermost magnetic fields effectually neutron stars. Intriguingly, in early 2021 Berkeley Lab astronomers reported the detection of unusual high-free energy X-ray emissions from a neutron star, which fit the bill for axions.

In another study, astronomers searched for show of dark matter dragging on black holes. The thought is that if a item type of ultralight boson (which includes axions) exists, clouds of these tiny particles should gather around blackness holes of a certain mass, and actually tiresome downward the rate at which they spin. Measuring the spin speed of black holes could then indicate the presence of nighttime matter.

Nada results aren’t void

The hunt for dark matter continues
The hunt for dark matter continues

Sadly enough, all of the experiments described above have either returned null results on dark matter, or are purely theoretical for now. Only non getting a signal doesn’t brand an experiment a total washout – null results are important to assist whittle away at that gigantic possibility space.

Each test searches for dark matter candidates within a sure mass range and with certain backdrop, and as we cross them off the list we’re getting ever closer to the truth. And it helps that many of the experiments are getting upgrades in the futurity that will make them ever-more sensitive.

In the meantime, brand new ideas are often proposed. In contempo years, scientists accept suggested that dark matter could take the form of superheavy gravitinos, d-star hexaquarks, or even a “dark fluid” with negative mass that permeates the universe.

Or of class, maybe it’s just a mathematical misunderstanding or if some other unseen and unknown force is creating these strange gravitational effects. Whatever information technology is, the chase for dark matter is far from over, and with so many scientists searching, we might before long brand 1 of the most important scientific discoveries of all fourth dimension – or realize we’ve been led on the greatest wild goose hunt ever.

Editor’s note:

This commodity was originally published on June, 2020. This revised and updated version, which covers the latest inquiry on dark matter, was published on April 20, 2021.