On April 3rd, 2018 we learned about

Strangely slow galaxy attributed to missing dark matter

Some missing dark matter may help confirm that this invisible substance exists. Dark matter is as-of-yet theoretical form of matter that doesn’t interact with light as we know it, making it invisible or dark to our various probes and sensors studying the universe. It does seem to interact with gravity like other matter does though, and so most of our ideas about it come from watching how normal objects move and react to the pull of something that’s otherwise undetectable. Of course, since as much as 80 percent of the mass of the universe is actually dark, studying what is does and doesn’t do isn’t easy.

Luckily, dark matter doesn’t seem to be equally distributed. Galaxy NGC 1052–DF2, a cluster of stars 65 million light-years away from Earth, may lack dark matter entirely. NGC 1052–DF2 looks pretty normal at any given moment, but the over time the motion of some bundles of stars, called globular clusters, has turned out to be too slow. For the amount of mass we can see, the galaxy should have around 60 billion times our own Sun’s mass in dark matter. The total mass of normal and dark matter would then require the globular clusters to move much faster than they’ve been observed. The orbital speeds that have been seen only make sense if there’s no dark matter there at all.

How could the dark matter be missing?

This is weird, to say the least. Galaxies are thought to form around the gravitational well created by clumps of dark matter, so this finding raises questions about how NGC 1052–DF2 ever came together in the first place. One possibility is that the dark matter has been stolen by some of its neighbors. Some galaxies have been found with extra dark matter, and so there’s a chance that NGC 1052–DF2 lost its invisible mass to the stronger gravitational pull of another galaxy.

The last option is that there was never dark matter in NGC 1052–DF2, or anywhere else for that matter. Some researchers suggest that the best way to make sense of the movement of large galaxies is with modified Newtonian dynamics, or MOND. In this model, the physics we’re used to just aren’t correct on the scale of a galaxy, and thus need adjusting so that they make sense with what we’ve observed. However, the weird speeds of NGC 1052–DF2 are strengthening doubts about MOND, since even modified dynamics should be consistent in every galaxy. It’s then easier to imagine one weird galaxy missing its dark matter than figuring out some other caveat to explain why the math behind MOND would need further adjustments in just this case.

Source: Dark matter is MIA in this strange galaxy by Emily Conover, Science News

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