Dark matter is one of the biggest mysteries in physics today. Based on observations from cosmologists, we know that all the matter we see around us — every proton, electron, and neutron — comprises just a tiny fraction of all the matter that exists in the universe. So what is all this other matter? Physicists theorize it must be a type of particle that we currently can’t directly detect, though we can see its effects. They call this hypothetical particle dark matter.
Studying very large galaxies is helpful in understanding dark matter as we know dark matter clusters around galaxies to form a halo. The gravitational effects of these massive halos are more obvious when the galaxy in question is a large one. So recently, the Hubble Space Telescope captured this image of the enormous galaxy cluster Abell 3827, which creates a strong gravitational lensing effect.
This galaxy cluster was the site of a debate over the nature of dark matter. In 2015, some scientists believed they observed dark matter interacting with other dark matter in this region when they saw a cloud of dark matter which was lagging behind the galaxy it surrounds. This means there would be a type of dark matter particle which is different from the standard view of dark matter, called the Lambda cold dark matter model.
However, this idea was eventually disproved when the same group of scientists made more observations in 2017, adding data from the Atacama Large Millimeter/submillimeter Array as well as the Very Large Telescope’s MUSE instrument to improve their model of the cluster.
This new set of observations “reveals an unusual configuration of strong gravitational lensing in the cluster core, with at least seven lensed images of a single background spiral galaxy,” the scientists wrote. “The new spectroscopic data enable better subtraction of foreground light, and better identification of multiple background images. The inferred distribution of dark matter is consistent with being centered on the galaxies, as expected by [the Lambda cold dark matter model].”
That means that the new data showed dark matter behaving as expected in the traditional view, and did not support the idea of self-interacting dark matter. Scientists continue to study dark matter using tools like the upcoming European Space Agency Euclid telescope to try to understand more about this mysterious phenomenon.