The Mysteries of Dark Matter

Dark matter just got even more puzzling.

This unidentified stuff, which makes up most of the mass in the cosmos, is invisible but detectable by the way it gravitationally tugs on objects like stars. Dark matter’s gravity can also bend light traveling from distant galaxies to Earth — but now some of this mysterious substance appears to be bending light more than it’s supposed to. A surprising number of dark matter clumps in distant clusters of galaxies severely warp background light from other objects.

This finding suggests that these clumps of dark matter, in which individual galaxies are embedded, are denser than expected. And that could mean one of two things: Either the computer simulations that researchers use to predict galaxy cluster behavior are wrong, or cosmologists’ understanding of dark matter is. Very high concentrations of dark matter can act like a lens to bend light and drastically alter the appearance of background galaxies as seen from Earth — stretching them into arcs or splitting them into multiple images of the same object on the sky. Judging by computer simulations of galaxy clusters, clumps of dark matter around individual galaxies that are dense enough to cause such dramatic gravitational lensing effects should be rare.

But telescope images told a different story. The researchers used observations from the Hubble Space Telescope to investigate 11 galaxy clusters from about 2.8 billion to 5.6 billion light-years away. In that set, the team identified 13 cases of severe gravitational lensing by dark matter clumps around individual galaxies. These observations indicate there are more high-density dark matter clumps in real galaxy clusters than in simulated ones. The simulations could be missing some physics that leads dark matter in galaxy clusters to glom tightly together. Scientists think the crux of the problem is more likely in computer simulations than in the nature of dark matter. A cluster of galaxies is a very dangerous place — busy with galaxies whizzing past one another, colliding and getting torn up. There’s awful physics that goes into predicting how many of these little lensed things they should find.

CITATIONS

M. Meneghetti et al. An excess of small-scale gravitational lenses observed in galaxy clusters. Science. Vol. 369, September 11, 2020, p. 1347. doi: 10.1126/science.aax5164.