Another Galaxy Without Dark Matter Dark matter is thought to account for about 85% of the Universe's total mass, yet there are galaxies that stubbornly refuse to conform to this rule. A team of astronomers led by researchers from Yale, using the Keck Observatory, has discovered a third such... galaxy: the faint dwarf DF9, which appears to lack invisible mass entirely. The concept of dark matter emerged in the 1970s, when Vera Rubin first convincingly demonstrated that galaxies are held together by some form of invisible mass. Since then, a wealth of indirect evidence—such as galactic halos and gravitational lensing—has accumulated. This makes the exceptions all the more intriguing. DF9 sits alongside two other similarly "empty" galaxies—DF2 and DF4—as part of an elongated chain of seven galaxies located 45 million light-years away; all of them appear to have originated from a single event. How was this determined? By analyzing stellar motion, the team estimated DF9's mass at approximately 100 million solar masses—a figure that matches the combined mass of its visible stars, gas, and dust. Had dark matter been present, the total mass would have been roughly a hundred times greater. The necessary precision was achieved using the KCWI spectrograph on the Keck telescope, an instrument specifically designed to detect extremely faint light sources. It appears that DF2, DF4, and DF9 formed during a high-speed galactic collision that stripped gas clouds away from their dark matter halos, allowing the new galaxies to coalesce from ordinary matter alone. Lead author Michael Keim notes that a chain of dark-matter-free galaxies has never been observed before. His supervisor, Pieter van Dokkum, adds that this provides compelling evidence that dark matter is a genuine physical substance rather than merely a "correction" to the theory of gravity.show more

Black Hole

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