About 65 million light-years from Earth, three galaxies don’t look quite right. They aren’t whirlpools bursting with light, but pallid objects dimly illuminated by a smattering of stars. “They have this ghostly appearance,” said Michael Keim (opens a new tab), an astrophysicist at Yale University. And this isn’t even the weirdest thing about them.
Dark matter is the invisible glue that keeps the largest structures in the cosmos together. It draws ordinary, visible matter toward it, allowing stars to form and keeping the stelliferous limbs of galaxies from flying off. “Almost all galaxies are dominated by dark matter,” said Shyam Balaji (opens a new tab), a theoretical physicist at King’s College London.
But these three diffuse galaxies, known as DF2, DF4, and DF9, have left astronomers stunned. After using a suite of telescopes to estimate their masses — using the motions of their stars as a proxy — the researchers found that all three have little dark matter. Instead, a pittance of ordinary matter alone must have forged their small selection of stars. How could these rule-breaking galaxies have formed?
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