The 11 biggest unanswered questions about dark matter | Live Science

In the 1930s, a Swiss astronomer named Fritz Zwicky noticed that galaxies in a distant cluster were orbiting one another much faster than they should have been given the amount of visible mass they had. He proposed than an

unseen substance, which he called dark matter, might be tugging gravitationally on these galaxies.

Since then, researchers have confirmed that they can find this mysterious material throughout the cosmos, and that it

is six times more abundant than the normal matter that makes up ordinary things like stars and people. Yet despite

seeing Dark matter throughout the universe, scientists are mostly still scratching their heads over it. Here are the 11

biggest unanswered questions about Dark matter.

What is DM?

The 11 biggest unanswered questions about dark matter | Live Science

First and perhaps most perplexingly, researchers remain unsure about what exactly dark matter is. Originally, some

scientists conjectured that the missing mass in the universe was made up of small faint stars and black holes, though

detailed observations have not turned up nearly enough such objects to account for Dark matter’s influence, as

physicist Don Lincoln of the U.S. Department of Energy’s Fermilab previously wrote. The current leading contender for

dark matter’s mantle is a hypothetical particle called a Weakly Interacting Massive Particle, or WIMP, which would

behave sort of like a neutron except would be between 10 and 100 times heavier than a proton, as Lincoln wrote. Yet,

this conjecture has only led to more questions ­ — for instance…

Can we detect dark matter?

The 11 biggest unanswered questions about dark matter | Live Science

If dark matter is made from WIMPs, they should be all around us, invisible and barely detectable. So why haven’t we

found any yet? While they wouldn’t interact with ordinary matter very much, there is always some slight chance that a

dark-matter particle could hit a normal particle like a proton or electron as it travels through space. So, researchers

have built experiment after experiment to study huge numbers of ordinary particles deep underground, where they are

shielded from interfering radiation that could mimic a Dark-matter-particle collision. The problem? After decades of

searching, not one of these detectors has made a credible discovery. Earlier this year, the Chinese PandaX experiment

reported the latest WIMP nondetection. It seems likely that Dark-matter particles are much smaller than WIMPs, or lack

the properties that would make them easy to study, physicist Hai-Bo Yu of the University of California, Riverside said.

Does DM consist of more than one particle?

The 11 biggest unanswered questions about dark matter | Live Science

Ordinary matter is made up of everyday particles like protons and electrons, as well as a whole zoo of more exotic

particles like neutrinos, muons and pions. So, some researchers have wondered if dark matter, which makes up 85

percent of the matter in the universe, might also be just as complicated. “There is no good reason to assume that all

the dark matter in the universe is built out of one type of particle,” physicist Andrey Katz of Harvard University said.

Dark protons could combine with dark electrons to form dark atoms, producing configurations as diverse and

interesting as those found in the visible world, Katz said. While such proposals have increasingly been imagined in

physics labs, figuring out a way to confirm or deny them has so far eluded scientists. [Strange Quarks and Muons, Oh

My! Nature’s Tiniest Particles Dissected]

Does dark matter exist in every galaxy?

The 11 biggest unanswered questions about dark matter | Live Science

Because it so massively outweighs ordinary matter, dark matter is often said to be the controlling force that organizes

large structures such as galaxies and galactic clusters. So, it was strange when, earlier this year, astronomers

announced that they had found a galaxy named NGC 1052-DF2 that seemed to contain hardly any dark matter at all.

“Dark matter is apparently not a requirement for forming a galaxy,” Pieter van Dokkum of Yale University told

Space.com at the time. However, over the summer, a separate team posted an analysis suggesting that van Dokkum’s

team had mismeasured the distance to the galaxy, meaning its visible matter was much dimmer and lighter than the

first findings and that more of its mass was in dark matter than was previously suggested.

Could dark matter have an electrical charge?

The 11 biggest unanswered questions about dark matter | Live Science

A signal from the beginning of time has led some physicists to suggest that DM might have an electrical

charge. Radiation with a wavelength of 21 centimeters was emitted by stars in the universe’s infancy, just 180 million

years after the Big Bang. It was then absorbed by cold hydrogen that was around at the same time. When this radiation

was detected in February of this year, its signature suggested that the hydrogen was much colder than scientists had

predicted. Astrophysicist Julian Muñoz of Harvard University hypothesized that DM with an electrical charge

could have drawn heat away from the all-pervasive hydrogen, sort of like ice cubes floating in lemonade, as he said at

the time. But the conjecture has yet to be confirmed.

Does dark matter actually exist?

The 11 biggest unanswered questions about dark matter | Live Science

Given the difficulties that scientists have faced trying to detect and explain DM, a reasonable questioner might

wonder if they’re going about it all wrong. For many years, a vocal minority of physicists have pushed the idea that

perhaps our theories of gravity are simply incorrect, and that the fundamental force works differently on large scales

than we expect. Often known as “modified Newtonian dynamics,” or MOND models, these suggestions posit that there

is no dark matter and the ultrafast speeds at which stars and galaxies are seen to rotate around one another is a

consequence of gravity behaving in surprising ways. “Dark matter is still an unconfirmed model,” wrote physicist Don

Lincoln in an explainer. Yet the detractors have yet to convince the larger field of their ideas. And the latest evidence? It

also suggests that it is real.

BY Cynthia Nwankwo

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