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?
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?
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?
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?
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?
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?
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