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19 Galaxies Are Apparently Missing Dark Matter. No One Knows Why.

A panoramic image shows the Milky Way, our home galaxy.
A panoramic image shows the Milky Way, our home galaxy.
(Image: © ESO/S. Brunier)

Nineteen newly discovered dwarf galaxies seem to be missing their dark matter, and physicists aren't sure why.

The find dramatically increases the number of galaxies that appear to be missing dark matter, the mysterious, invisible stuff that exerts gravitational pull, yet emits no light. Dark matter is thought to be a key ingredient in galaxy formation, with its gravity pulling together atoms of gas to form galaxies. We can tell dark matter is present in a galaxy because it makes the matter in that galaxy swirl faster than it would if the matter we see made up the galaxy's whole mass. This faster swirling has shown up in every galaxy that could be precisely measured. Recently, however, researchers have found that certain small galaxies, now including these 19, behave as if they're dominated by baryons — the particles that make up ordinary matter. The evidence for their unseen halos of dark matter is missing.

Kyle Oman, an astrophysicist at Durham University in the United Kingdom, who wasn't involved in this discovery, said that these galaxies comprise the longest list of apparently dark matter-free objects yet reported. But they aren't the first.

Related: The 11 Biggest Unanswered Questions About Dark Matter

The most widely reported discovery of a galaxy that seemed to lack dark matter came in March 2018. A team of astrophysicists led by Pieter van Dokkum, an astrophysicist at Yale University, showed that the average speed of globular clusters in galaxy NGC 1052–DF2 matched a baryons-only galaxy model, though many questioned the validity of the result, as Live Science reported.

Even as that story played out, however, additional  teams found galaxies that seemed to lack dark matter. Those researchers used a different method, which singles out the motion of hydrogen atoms in a distant galaxy — the galaxy's "H1 rotation curves" — to show that the clouds of gas rotated as if they were not under the influence of dark matter. Oman worked on one such paper, which was published Sept. 25 in The Astrophysical Journal Letters.

The newest paper, published Nov. 25 in the journal Nature Astronomy, identified the 19 dark matter-free galaxies using the same method.

"These H1 rotation curves are more accurate" than the method used by van Dokkum's team, said Till Sawala, an astrophysicist at the University of Helsinki. However, he said, there are still "systematic uncertainties" in the measurements that are yet to be resolved.

For instance, if the angle of the host galaxy relative to Earth is incorrectly measured, that can screw up the calculations, Oman said. And events like supernovas can speed up the gas in a normal galaxy, creating rotation curves that look from Earth like the galaxies in this new paper, Sawala said. Either way, more follow-up work is needed to confirm the dark matter-free claim, all the experts (including the study authors) said.

If it does turn out that certain galaxies do lack the normal amount of dark matter, however, that's a problem for current theories of how the universe formed.

Physicists  explain how the universe formed and behaves using a model known as Lambda cold dark matter (ΛCDM). It describes three key features of the universe: the cosmological constant (Λ), dark matter and dark energy.

ΛCDM explains the way galaxies form, Sawala said, and it can't easily explain how these particular galaxies could have formed without dark matter.

Some of the examples that have turned up, Oman said, could be explained by the ΛCDM. For instance, dwarf galaxies in the middle of dense galaxy clusters have many other gravity sources around to strip away their dark matter. But in this paper, Oman said,  some of the dark matter-free galaxies are solitary, far from any other gravity source.

"That's a challenge," Oman said.

Some researchers have presented the evidence of dark matter-free galaxies as the knockout punch in a battle between ΛCDM and another set of theories known as Modified Newtonian Dynamics (MOND). MOND theories reject dark matter in favor of tweaks to the physics of gravity. Since gravity should act everywhere in the universe, MOND would predict that what we call dark matter should also be everywhere, including every galaxy. But if these galaxies violate MOND, they violate ΛCDM as well, so it's not really a knockout punch to MOND, Sawala said.

Physicists said the only way to figure out what's going on is to study these galaxies in much more detail using different tools and confirm that what seems to be happening there is really happening.

Originally published on Live Science.

  • mpc755
    Dark matter is a supersolid that fills 'empty' space and is displaced by ordinary matter.

    The supersolid dark matter displaced by a galaxy pushes back, causing the stars in the outer arms of the galaxy to orbit the galactic center at the rate in which they do.

    Displaced dark matter is curved spacetime. More correctly, what is referred to geometrically as curved spacetime physically exists as displaced dark matter. Curved spacetime is a geometrical representation of gravity. Displaced supersolid dark matter is the physical manifestation of gravity.

    The supersolid dark matter displaced by the quarks the Earth consists of, pushing back and exerting pressure toward the Earth, is gravity.

    Displaced supersolid dark matter is gravity.

    The reason for the mistaken notion the galaxies are missing dark matter is that the galaxies are so diffuse that they don’t displace the supersolid dark matter outward and away from it to the degree that the dark matter is able to push back and cause the stars far away from the galactic center to speed up. The galaxies are too diffuse to displace the supersolid dark matter to such an extent that it causes the light to lense as it passes through the galaxies.
    Reply
  • johnasb
    Or, given that multiple dimensions are proven, would it not be necessary that there is an interdimensional force that bounds them relative to each other and within their range. Perhaps, akin to what binds entangled particles beyond the limits of space is sufficient in larger black holes to entangle the stars in a consistent orbit. Will be interesting to see if this remains relative to galaxy size and can then be measured.
    Reply
  • mercster
    "No one knows why"? I know why, it's because "dark matter" and "dark energy" are made-up constants that need to be plugged into the scientists' equations to account for the data that they see. It's the same as "aether", which was made up to try and explain certain phenomena, before Einstein came along. Anyone who doesn't see this, has more faith than most religious people.
    Reply
  • Runesmith
    mercster said:
    "No one knows why"? I know why, it's because "dark matter" and "dark energy" are made-up constants that need to be plugged into the scientists' equations to account for the data that they see. It's the same as "aether", which was made up to try and explain certain phenomena, before Einstein came along. Anyone who doesn't see this, has more faith than most religious people.

    I fully agree. The problem is that people are so focused on "finding dark matter" rather than looking at whether the current theory of gravity requires modifications. Sadly, the only focus on a modified theory of gravity comes from "YouTube physicists" who have never published a paper and/or have no idea what they are talking about. This has tainted any examination of theory of gravity with a "lunatic fringe" tinge, probably preventing people with actual knowledge from venturing there.
    Reply
  • Howard Jeffrey Bender
    My novel idea is that galaxy centers are branes (a dimension from String Theory) rather than Black Holes. I have several videos talking about Dark Energy, Dark Matter, Quantum Entanglement, Creating Universes, etc., all using concepts from String Theory. Focusing on dwarf galaxies for now, check out this video uuG4yy-vW84View: https://www.youtube.com/watch?v=uuG4yy-vW84
    Reply
  • Howard Jeffrey Bender
    Runesmith said:
    I fully agree. The problem is that people are so focused on "finding dark matter" rather than looking at whether the current theory of gravity requires modifications. Sadly, the only focus on a modified theory of gravity comes from "YouTube physicists" who have never published a paper and/or have no idea what they are talking about. This has tainted any examination of theory of gravity with a "lunatic fringe" tinge, probably preventing people with actual knowledge from venturing there.
    I'm a YouTube physicist. Tell me what's wrong with either of these. No one has so far. epk-SMXbu1cView: https://www.youtube.com/watch?v=epk-SMXbu1c 24WyRKT8t4w:2View: https://www.youtube.com/watch?v=24WyRKT8t4w&t=2s
    Reply