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The uncertainty of science: Using the Keck telescope in Hawaii, astronomers have discovered a galaxy that apparently has no dark matter, the third such galaxy discovered.

The galaxy, dubbed DF9, is shown to the right. It is part of a region that includes the other two galaxies with no dark matter, suggesting all three were formed in linked circumstances.

The team used Keck Observatory’s Keck Cosmic Web Imager (KCWI) to measure the motions of stars inside DF9 by analyzing the light emitted across different wavelengths. Those measurements revealed that DF9 has a mass of only about 100 million Suns, consistent entirely with the galaxy’s visible matter. If the galaxy contained a typical amount of dark matter, astronomers would expect it to be about 100 times more massive.

Dark matter was invented by astronomers in the 1960s to explain the inexplicable fast rotation of stars in the outer fringes of every galaxy they looked at, much faster than accounted for by the visible mass of each galaxy. That fast rotation suggested there was a lot more matter there that could not be seen, as much as one quarter of all mass according to some models. This unseen matter was then labeled “dark matter”, though even three-quarters of a century later it still remains undetected directly.

That these galaxies have no dark matter poses a difficult problem. At present the scientists are trying to come up with scenarios for creating these galaxies without it, but I suspect they will be hard-pressed to come up with any theory that convinces anyone.

I can’t help wondering if the problem isn’t the fast rotation of the stars, but the observations themselves. Maybe there is something we are missing or are unable to detect, because of the vast distances and faint nature of the light. Either way, dark matter, there or not, remains one of the big fundamental mysteries of cosmology.