Free Astronomy Magazine January-February 2021

NOVEMBER-DECEMBER 2020 25 ASTRO PUBLISHING T his NASA/ESA Hubble Space Telescope image shows the massive galaxy cluster MACSJ 1206. Overlaid on the image are small-scale concentrations of dark matter (represented in this artist’s impression in blue). [NASA, ESA, G. Caminha (Uni- versity of Groningen), M. Meneghetti (Observatory of Astrophysics and Space Sci- ence of Bologna), P. Natarajan (Yale University), the CLASH team, and M. Kornmesser (ESA/Hubble)] stars, dust, and gas together in a galaxy. This mysterious substance makes up the bulk of a galaxy’s mass and forms the foundation of our Universe’s large-scale structure. Be- cause dark matter does not emit, ab- sorb, or reflect light, its presence is only known through its gravita- tional pull on visible matter in space. Astronomers and physicists are still trying to pin down what it is. Galaxy clusters, the most massive and recently assembled structures in the Universe, are also the largest repositories of dark matter. Clusters are composed of individual member galaxies that are held together largely by the gravity of dark mat- ter. “Galaxy clusters are ideal labo- ratories in which to study whether the numerical simulations of the