Observations of individual galaxies, of clusters of galaxies, and of the cosmic microwave background suggest that most of the universe’s mass is in the form of dark matter – matter that does not emit any light, but which can be detected through its gravitational effects. The best dark- matter candidates are exotic elementary particles that have yet to be discovered with accelerator experiments, and that interact very weakly with familiar particles and with themselves (only through gravity and the weak nuclear force).

But astronomers can probe the spatial distribution of dark matter through gravitational lensing – how dark matter’s gravity distorts the images of background galaxies. Just as the images of pebbles at the bottom of a pool appear distorted due to the refraction of light through water, the light of distant galaxies passes through the gravitational influence of intervening clusters, resulting in warped or stretched images. Using HST’s sharp view, astronomers employ this lensing technique to reconstruct the large-scale, three-dimensional distribution of the dark matter responsible for the distortions. In one study, astronomers constructed the distribution of dark matter (in one direction) 3.5, 5.0, and 6.5 billion years ago. They found that dark-matter clumping becomes more pronounced over time.

Astronomers have also used HST to observe the matter’s distribution in the collision of two clusters of galaxies, one seen pole-on and the other from a direction perpendicular to the collision axis. In the latter case in particular (known as the Bullet Cluster), a combination of HST and Chandra X-ray observations suggests that the hot gas in the two clusters interacts strongly, producing a shock front, while the matter completely separates from the gas. This is precisely what we expect from very weakly interacting particles, which is what leading theories predict for dark matter.

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