A distant galaxy doesn't seem to contain any dark matter, and that's really weird
Billions of years ago, our Milky Way galaxy pulled itself together.
Our home condensed from gas that gathered in our small part of the universe, swirled together thanks to gravity provided by dark matter -- the mysterious substance that makes up 85 percent of matter in the universe and doesn't interact with regular matter.
That dark matter allowed the Milky Way's gas -- i.e. the "normal" matter -- of our galaxy to condense and form stars.
Researchers have thought for some time that, for the most part, galaxies need to form with the help of dark matter because of the extra gravity the mysterious, plentiful substances provides.
But now we know of a galaxy located about 65 million light-years away in which that model doesn't hold.
According to a new study published in the journal Nature this week, at least one galaxy, named NGC 1052–DF2, is devoid of dark matter.
“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” astronomer Pieter van Dokkum, lead author of the new study, said in a statement.
“This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work..."
In photos, the galaxy -- which is about the size of the Milky Way but contains only 1/200th of the stars, according to the study -- looks like a somewhat diffuse blob that's so transparent that you can actually see distant galaxies beyond it.
While finding this kind of galaxy is bizarre, it's not totally unheard of. Our understanding of dark matter and regular matter leaves room for this kind of galaxy.
For example, in 2016, a study detailed the discovery of a galaxy made almost entirely of dark matter. (Think of it as the Bruce Willis to NGC 1052–DF2's Samuel L. Jackson in Unbreakable.)
That said, it's still an oddball.
Van Dokkum and his colleagues have come up with a couple explanations for why NGC 1052–DF2 lacks any measurable dark matter.
It's possible that some kind of violent interaction between galaxies could have left NGC 1052–DF2 devoid of dark matter. The study also speculates that the galaxy could have formed when matter was pushed out from a separate galaxy, creating a clump of gas that could form another galaxy without much dark matter.
"It makes sense that that could happen sometimes, that you could have circumstances such that gas is coming together, maybe from an outflow or from some gas falling into a galaxy, and while that gas is coming together through its own gravity, it could fragment and form stars. So, it's not inconceivable," astrophysicist Katie Mack, who wasn't involved in the new study, said in an interview.
"It's a very unusual kind of object."
The new discovery could also answer some serious questions about the nature of dark matter on a large scale.
Namely, the new study -- if correct -- shows that dark matter is a real substance, which is a matter of some debate in the astrophysics community.
Some scientists have suggested that what we call dark matter is actually just how regular matter functions on a grand, cosmic scale. In those models, dark matter is basically limited to an effect that happens when enough regular matter is clumped into a part of space.
"It's a really interesting object because it's a way of demonstrating that dark matter is a real thing by not seeing it in this particular case," Mack said.
"If it [dark matter] really is a consequence of regular matter, then we should see it every time there’s enough regular matter. If we don’t see it when there’s a lot of regular matter, then it really has to be something else that’s not 100 percent always the consequence of regular matter existing.”
Van Dokkum and his colleagues checked out the galaxy using the Dragonfly Telescope Array in New Mexico, which is able to find relatively dark structures out in the universe.
They also performed follow-up observations using the Hubble Space Telescope and Gemini Observatory to learn all they could about the newfound galaxy.
While NGC 1052–DF2 had been observed before, it looked very different when seen through Dragonfly's mechanical eyes.
“I spent an hour just staring at the Hubble image,” van Dokkum said in the statement.
“It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and say, ‘I’ve never seen that before.’ This thing is astonishing, a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”
If you were to find yourself standing on a planet orbiting one of the stars in the faraway galaxy, you might look out into your night sky and see star clusters around you.
But unlike our skies on Earth, the night skies of planets in NGC 1052–DF2 wouldn't be dominated by the bright, cloudy arms of a spiral galaxy.
"It wouldn't look like a big... trail of stars across the sky the way we see the Milky Way," Mack said.
"There would probably be a sort of diffuse glow in the sky from the rest of the galaxy that would maybe be stronger in one direction than in the opposite direction. It's very dim, so it might just look like a very starry sky."
Miriam Kramer worked as a staff writer for Space.com for about 2.5 years before joining Mashable to cover all things outer space. She took a ride in weightlessness on a zero-gravity flight and watched rockets launch to space from places around the United States. Miriam received her Master's degree in science, health and environmental reporting from New York University in 2012, and she originally hails from Knoxville, Tennessee. Follow Miriam on Twitter at @mirikramer.
