Rubin calculated that visible matter in the galaxies she observed must account for just 10% of their mass, and when revisiting Zwicky's findings from around four decades before, she discovered that a similar ratio of seen and unseen matter binds the Coma Cluster. This meant that there had to be a huge amount of invisible matter in the outer regions of galaxies away from dense stellar populations. This was odd, as the visible mass of these galaxies shouldn't have enough gravitational influence to keep stars moving rapidly in sparsely populated outer regions in place. Rubin saw that the stars at the edge of spiral galaxies far from their dense centers were moving as fast as stars closer to the galactic heart, according to the American Museum of Natural History. While studying the rotational dynamics of galaxies, Carnegie Institution astronomer Vera Rubin became the next scientist to infer the presence of dark matter with observations that helped to cement it as an accepted element of the universe. Zwicky suggested that an as-yet-unobserved type of mass - "dunkle Materie," or "dark matter" - might explain this disparity, but the concept wouldn't be widely accepted for decades to come, until after his death in 1974. Zwicky, later dubbed "the father of dark matter," found that single galaxies in this cluster were moving too fast for the cluster to remain together based on the gravity of the visible matter alone.
The first hints of dark matter were observed in 1933 when California Institute of Technology astronomer Fritz Zwicky used the Mount Wilson Observatory to measure the visible mass of the Coma Cluster of galaxies. It is through the interaction with gravity that astronomers were first able to discover dark matter and, later, accurately map it. (Image credit: NASA/ESA/JPL-Caltech/Yale/CNRS)Īlthough dark matter does not interact strongly via electromagnetism, it does interact with another fundamental force: gravity. Related: How does astronomy use the electromagnetic spectrum? How do scientists know dark matter is there?Ī halo of dark matter, represented in purple, surrounds the inner region of a galaxy cluster called Abell 1689. This means dark matter can't be seen in traditional ways that rely on electromagnetic radiation. Scientists know that dark matter is distinct from ordinary baryonic matter because whatever particles dark matter comprises either don't reflect, absorb or emit electromagnetic radiation or if they do interact with light, they do so incredibly weakly. Even dark clouds of cosmic gas that do not shine brightly absorb photons of light at certain wavelengths, so they can be seen by their interaction with light. When electromagnetic radiation, including visible light, falls on baryonic objects, they absorb photons and reemit them, or simply reflect them, so these objects can be seen. Most of these cosmic objects (as well as Earthbound objects, such as tables, cars and cats) can be seen because baryons interact with the electromagnetic force, one of the universe's four fundamental forces. According to the Swinburne Centre for Astrophysics and Supercomputing, cosmic objects made of baryonic matter include clouds of cold gas, planets, comets and asteroids, stars, neutron stars and even black holes. Everything we see around us on a day-to-day basis is made up of "ordinary" matter, known as baryonic matter, meaning it's composed of baryons (such as protons and neutrons).
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