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Dark Matter Clump Furrows Brows

Dark matter doesn't usually collide much with itself or with ordinary matter, but it appears to be uncharacteristically clumping in the galaxy cluster A520. John Matson reports

Dark matter. It’s hard to see, it’s hard to study and it just won’t behave. There’s plenty dark matter around. It’s just that no one knows what it is. It only makes its presence felt through its gravitational pull.

But astronomers have figured out ways to ID dark matter. One tactic is called gravitational lensing. Dark matter’s gravitational pull bends light, so clumps of dark matter distort the appearance of galaxies in the background. Researchers measured those distortions with the Hubble Space Telescope. And they recently mapped out the dark matter in a galaxy cluster called A520. Which is actually several galaxy clusters, all smashing together.

The problem was, the dark matter in A520 wasn’t where it ought to be. Past studies have shown that clouds of dark matter pass cleanly through one another when galaxy clusters merge. That’s because dark matter doesn’t collide much with itself or with ordinary matter. But in this case the dark matter seemed to bunch up in the middle, as if it was sticking together. The study appears in the Astrophysical Journal. [M. James Jee et al., "A study of the dark core in A520 with the Hubble Space Telescope: The mystery deepens"]

Why should dark matter behave so differently in different galaxy clusters? On that front, astronomers are still in the dark.

—John Matson

[The above text is a transcript of this podcast.]

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