Something doesn鈥檛 look quite right in clusters of galaxies. The structure of the dark matter that suffuses these enormous clumps of matter is far different to what聽we would expect, and none of our current models can explain this behaviour.
One way to measure dark matter in distant galaxies or clusters of galaxies is by analysing how they affect the light coming from objects behind them. When light passes a massive object, such as a galaxy full of dark matter, the gravity of that galaxy can bend the light in what is known as聽gravitational lensing.
Priyamvada Natarajan at Yale University and her colleagues examined the lensing data from 11 galaxy clusters to determine the structure of the dark matter within them. They then compared that data with 25 simulated galaxy clusters.
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鈥淵ou can think of a cluster as a big mountain range with a lot of little peaks that are associated with the cluster鈥檚 galaxies,鈥� says Natarajan. The cluster itself can act as a gravitational lens, as can each galaxy within it. 鈥淎round these little peaks of dark matter around the galaxies, you could see little events of extra lensing, and the frequency of these small-scale lensing events is much higher than we thought,鈥� she says.
Those relatively small lensing effects were 10 times stronger than what was predicted by the simulations, and even after Natarajan and her team tested their results against a range of possible models of dark matter, it wasn鈥檛 clear where that discrepancy came from. 鈥淣othing that is currently in our arsenal of ideas can easily fix this, so I think it鈥檚 very exciting,鈥� says Natarajan.
There are two possibilities: either there is some error in the simulations, which are state of the art and consistent with other observations, or dark matter behaves differently from how physicists thought it did. Either way, the discrepancies in these lensing events hint that we are missing something big about our universe.
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