Shining a Light on Dark Matter

Hubble’s observations help astronomers uncover the underlying structure of the universe.

A cluster of galaxies fills the frame. A purple glow around the largest concentrations of galaxies indicates the distribution of dark matter.

More than 80% of the universe is made of stuff we have never seen. These ghostly forms of energy and matter are only detectable by the effects they have on the stuff we can see. The invisible form of matter, called dark matter, makes up roughly 30% of the universe’s total mass. Its gravity drives normal matter (gas and dust) to collect and build up into stars, galaxies, and massive galaxy clusters. Although astronomers cannot see dark matter, they can detect its influence by observing how its gravity bends and distorts light from more-distant objects, a phenomenon called gravitational lensing. 

Lower left corner: Hubble sits looking toward the upper-right corner where there is a spiral galaxy. Between the two is an image of a large galaxy cluster. Lines drawn from the spiral at upper-right to Hubble illustrate the gravitational lens created by the galaxy cluster.
Large galaxy clusters contain both dark matter and normal matter. The immense gravity of all this material warps the space around the cluster, causing the light from objects located behind the cluster to be distorted and magnified. This phenomenon is called gravitational lensing. This sketch shows paths of light from a distant galaxy that is being gravitationally lensed by a foreground cluster.

By looking at the area around massive galaxy clusters, which contain both normal and dark matter, astronomers can identify warped background galaxies gravitationally lensed by the cluster and reverse-engineer their distortions. Mathematical models of these results shed light on the location and properties of the densest concentrations of matter in the cluster, both visible (normal) and invisible (dark). The universe appears to have about five times more dark matter than regular matter, and its structure may center on an immense network of dark matter filaments that stretch between galaxies and grow over time. Areas where concentrations of these filaments intersect also hold massive visible structures like galaxy clusters. 

two Hubble images of galaxy cluster Cl 0024+17 (ZwCl 0024+1652), with right image shaded to illustrate dark matter
This view of the massive galaxy cluster Cl 0024+17 (ZwCl 0024+1652) reveals the bent and amplified light of distant galaxies. The left view is in visible light with odd-looking blue arcs appearing among the yellowish galaxies. These are the magnified and distorted images of galaxies located far behind the cluster. The right image holds added blue shading that indicates the location of invisible dark matter. The shape and position of the gravitationally lensed galaxies we see in the left-hand image, mathematically requires the presence of this dark matter. 
NASA, ESA, M.J. Jee, and H. Ford (Johns Hopkins University)
A cluster of galaxies fills the frame. A purple glow around the largest concentrations of galaxies indicates the distribution of dark matter.
This is the Hubble Space Telescope image of the inner region of Abell 1689, an immense cluster of galaxies located 2.2 billion light-years away. Dark matter in the cluster is mapped by plotting the plethora of arcs produced by the light from background galaxies that is warped by the foreground cluster's gravitational field. Dark matter cannot be photographed, but its distribution is shown in the blue overlay. The dark matter concentration and distribution is then used to better understand the nature of dark energy, a pressure that is accelerating the expansion of the universe. The imaging data used in the natural-color photo was taken in 2002 with Hubble's Advanced Camera for Surveys.
NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), and T. Broadhurst (Tel Aviv University)

Because these elusive filaments are so hard to find, astronomers have turned to similar forms on Earth for clues of where to look. A single-celled slime mold, called Physarum polycephalum, builds complex filamentary networks in search of food, finding near-optimal pathways to connect different locations. Astronomers using Hubble data designed a computer algorithm inspired by the slime mold’s behavior, to simulate the growth of dark matter filaments. The simulation resulted in a three-dimensional computer model of estimated locations of the cosmic web’s filamentary structure.  

Illustration: Purple filaments fill the view against a black background. They represent the growth of slime mold.
Researchers turned to slime mold, a single-cell organism found on Earth, to help them build a map of the filaments in the local universe (within 500 million light-years from Earth) and find the gas within them. They designed a computer algorithm inspired by the organism's behavior and applied it to data containing the positions of 37,000 galaxies ("food" for the slime mold) mapped by the Sloan Digital Sky Survey. The algorithm produced a three-dimensional map of the underlying cosmic web's intricate filamentary network, the purple structure in the image. The research team noted a striking similarity between how the slime mold builds complex filaments to capture new food, and how gravity, in shaping the universe, constructs the cosmic web strands between galaxies and galaxy clusters. Then, after using their algorithm to find the cosmic web filaments, the astronomers used archival observations from the Hubble Space Telescope to detect and study the gas permeating the web.
NASA, ESA, and J. Burchett and O. Elek (UC Santa Cruz)
Caption: Astronomers using Hubble found small, dense concentrations of dark matter that bend and magnify light much more strongly than expected. 
NASA's Goddard Space Flight Center; Lead Producer: Paul Morris  

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