WE SEE its belongings in how stars move inside cosmic systems, and how universes move inside world groups. Without it, we can’t clarify how such enormous assortments of the issue came to exist, and absolutely not how they hang together today. However, what it is, we don’t have the foggiest idea.

Welcome to probably the greatest puzzle known to mankind: what makes up a large portion of it. Our best estimations show that exactly 85 percent of all issue in our universe comprises of “dim issue” made of something that isn’t iotas. Immense underground analyses worked to get looks at dim issue particles as they go through Earth have seen nothing. Molecule crushing examinations at the Large Hadron Collider, which we trusted would make the dull issue, haven’t – at any rate similar to we can tell. The chase for the dim issue was never expected to be simple. Be that as it may, we didn’t anticipate it to be this hard.
Dim issue’s no-show implies that numerous potential clarifications for it that individuals like me supported only ten years prior have currently been precluded. That is driving us to profoundly return to suppositions about the idea of the dim issue, yet in addition to the early history of our universe. This is the most recent turn in a long-running adventure: our inability to distinguish the particles that make up dull issue proposes that the start of the universe may have been altogether different from what we envisioned.

How about we start with what we do think about this substance – or maybe substances. The dim issue isn’t a well-known nuclear issue, or any of the intriguing types of the issue made at the Large Hadron Collider, covered underground close to Geneva, Switzerland, or at other molecule quickening agents. It doesn’t apparently cooperate with itself, or with the conventional-issue, aside from by means of gravity. It can go through strong articles like an apparition and doesn’t discharge, retain, or mirror any effectively quantifiable amounts of light.

It is undetectable, or if nothing else about so. However, without a dim issue, it is impossible that we would be here. As cosmic systems and universe groups were developed, dull issues assumed the job of the platform: it assembled into colossal mists whose gravity pulled in and pulled together the nuclear issue that would look at last structure the glowing piece of worlds. Without the gravity of dull issue holding stars set up, they would fly outwards, now and again, getting away into intergalactic space. Numerous worlds would essentially crumble.

We see dim issue’s engraving from numerous points of view, as well, for instance, in how a cosmic system group’s gravity redirects light that passes it. Maybe the best proof of just for dim issue’s presence originates from temperature designs saw in the infinite microwave foundation,

The radiation left over from the enormous detonation. Estimations of this radiation give us a guide of how the matter was appropriated all through our universe, just two or three hundred thousand years after its start. This guide discloses to us that our universe was exceptionally uniform in its childhood, with just the littlest varieties in thickness. Without assistance from dull issue, it’s absolutely impossible that these thickness varieties could have developed quick enough to frame the systems and other enormous structures of the present universe.

Ten years or all the more prior, numerous physicists, including me, thought we recognized what dull issue was probably going to comprise of: pitifully connecting monstrous particles or WIMPs. As their name recommends, these are moderately substantial particles that, other than gravity, just cooperate by means of the powerless atomic power, which likewise oversees sub-nuclear procedures, for example, radioactive beta rot. Weaklings appeared to be convincing in light of the fact that we could see how they would have been made in the early universe.
During the first millionth of a second or so after the big bang, all of the space was filled with a hot, dense plasma in which all sorts of particles, from photons and electrons to top quarks and Higgs bosons, were constantly being created and destroyed. As space expands, however, the temperature of the plasma steadily drops. Eventually, it can’t supply the energy required to make heavier particles, and their production stops.
When this happens to a species of particle, most are destroyed – annihilated – and converted into other forms of energy. How many survive depends on how and how often the particles interact.
This leads us to a happy coincidence: for a particle species to emerge from the big bang with an abundance equal to that of dark matter today, it must have interacted through a force about as powerful as the weak nuclear force.
A stronger force would have caused too many particles to be destroyed, while a feebler force would have allowed too many to survive.
Rather like the temperature of Goldilocks’s porridge, the strength of the weak force seems just right to explain how dark matter came to be formed in the heat of the big bang.
But that story now seems a fairy tale rather. If dark matter does consist of WIMPs, we can estimate how much it should interact with ordinary atomic matter via the weak force, and so design experiments to detect it. These experiments, housed in deep underground laboratories to avoid the constant bombardment of cosmic radiation, started out small, deploying detectors of only a few kilograms of crystalline materials such as germanium, calcium tungstate or sodium iodide, sensitive to the light, heat and electric charge that would be produced in collisions of WIMPs with normal matter.
Over the past two decades, the size and sophistication of these experiments have hugely increased. The latest iterations are enormous, deploying anything up to tonnes of liquid xenon as their detectors. These experiments – XENON1T under the Gran Sasso mountain in Italy (pictured, right), LUX in South Dakota, and PandaX-II in Sichuan, China – are each roughly 11,000 times as sensitive as the most sophisticated dark matter detectors operating in 2007.
But they too have failed to turn up WIMPs. The only experiment that even claims to have detected anything resembling dark matter goes by the name of DAMA. Most researchers think the signal is picked up is almost certainly produced by something else: a long list of other experiments have searched for the kinds of WIMPs that could have made it, but have seen nothing.
The only other possible piece of evidence we have for WIMPs comes in the form of a strange gamma-ray signal seen emanating from the center of the Milky Way. My collaborators and
I spotted this signal in data from NASA’s Fermi space telescope more than a decade ago. It took years for us to convince most people that it was real. We continue to debate whether these gamma rays are produced by dark matter, or by something else, such as a group of thousands of rapidly spinning neutron stars. At the moment, we just can’t be sure.
One possibility is that dark matter could interact with other forms of matter and energy even less than we had imagined – perhaps only through gravity or some force so feeble that we haven’t even discovered it yet. Such a particle would be even more difficult to detect in underground experiments or to produce with particle accelerators.
The problem is that such non-interacting particles would probably survive the big bang in vast numbers, and wildly exceed the abundance of dark matter in our universe today. But if they interact rarely enough, perhaps these particles were never produced in great quantities in the first place, instead of building up an appreciable abundance only gradually over the first fraction of a second of cosmic history.
It could be, too, that dark matter is just one of several kinds of particles that almost never interact with any known forms of matter and energy. This “hidden sector” of particles would involve forces and interactions that we have never observed, and that allows the dark matter to evolve in a wide variety of ways. These interactions may have depleted the amount of dark matter, without leading to any appreciable interactions with ordinary matter.
The hidden-sector particles might become bound to each other, forming dark nuclei or dark atoms. One day, we could even discover something like a periodic table of the hidden sector elements. For that reason, of all the plausible ideas about dark matter that have grown in popularity in recent years, this is perhaps my favorite.
Many of these alternative dark matter candidates call for experiments very unlike those designed to hunt for WIMPs. One example is the Axion Dark Matter Experiment, ADMX, based at the University of Washington in Seattle and managed by scientists at my institute, Fermilab. It uses powerful magnetic fields to try to convert the one hypothesized type of ultra-light dark matter particle, axions, into photons.
Some physicists are trying to produce dark matter using particle beams originally designed to study neutrinos. Others are designing tunable electronic circuits that could pick up signals of dark matter waves, much like a radio picks up electromagnetic waves consisting of photons. There are even ideas involving gravitational wave detectors. These ideas may not seem to have much in common, but they are all motivated by testing previously overlooked possibilities for dark matter.
There is an even more dramatic possibility that many cosmologists are considering. Our surprise at dark matter’s no-show is based on our current understanding of the early universe. Maybe we haven’t seen the particles because dark matter is different from what we had expected – or perhaps because the universe’s first moments were.
The amount of dark matter that was created in and survived the big bang depends on how our universe evolved during its hot and volatile youth. We know a great deal about most of our universe’s 13.8-billion-year history, but we have no direct observations that enable us to study the first fraction of a second, the window in which dark matter is thought to have formed.
One possibility my colleagues Hooman Davoudiasl and Sam McDermott and I have investigated is that our universe experienced a brief period of hyper-fast expansion during this era. We already think it did something similar right at the beginning, in an event known as cosmic inflation.
Another – somewhat less explosive – burst of expansion may have occurred somewhat later as well, still within the first part of a second of cosmic history. It would have diluted the amount of dark matter in the early universe, and thereby changed our expectations for how strongly this substance should interact – and how difficult it should be for us to detect.
Alternatively, there may have been a population of particles that decayed at some point in the early universe, disappearing and creating dark matter. Dark matter particles created in this way could be extremely feebly interacting, explaining why they have gone undetected for so long.
A third possibility is that our universe
went through an abrupt change during its
First moments – not merely a steady cooling, but a total phase transition. We already know of two such transitions in which the nature of particles and their interactions changed within the universe’s first second, what is known as the QCD and electroweak phase transitions. But there may have been others. A phase transition in dark matter interactions could have influenced how dark matter formed in the early universe, again altering our expectations for the kinds of experiments that might detect it today.
It is too early to say whether the right answer is one, some, or none of the above possibilities. Perhaps an experimental breakthrough will change the game yet again. But the stubborn elusiveness of dark matter has left many physicists and cosmologists surprised and confused. In droves, we are returning to our chalkboards, revisiting and revising our assumptions – and with bruised egos and a bit more humility, desperately attempting to find new ways to make sense of a very dark and hidden universe.

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