Why the universe's most massive objects are actually invisible
In short: The Milky Way is filled with dark matter, a substance that dominates the universe's contents. While we cannot see it directly, its presence is betrayed by the way it pulls on everything else, from individual galaxies to the most massive clusters in the cosmos.
The mystery of dark matter began in 1933 with Fritz Zwicky. While studying the Coma cluster of galaxies, Zwicky noticed the relative velocities of the galaxies within the cluster. He realized that, without some unseen matter to provide extra gravity, the cluster would simply fly apart. At that time, he estimated that dark matter must outnumber visible matter by ten times. Zwicky's original calculation has since been largely validated by modern observations. His initial findings were based solely on the relative velocities of cluster galaxies. Zwicky's groundbreaking work relied on the measurements of relative velocities of cluster galaxies. His estimate that dark matter outweighs visible matter tenfold remains largely accurate today.
Decades later, the evidence moved from entire clusters to individual galaxies. Astronomer Vera Rubin studied the Andromeda galaxy, a neighbor of the Milky Way, by observing the radial velocities of ionized hydrogen clouds in its nebulae. She discovered that the rotation curve of the galaxy did not decrease as distance from the center increased. This was later confirmed by radio astronomer Morton Roberts, who measured the velocities of atomic hydrogen clouds that extended even further. Rubin's observations focused on bright nebulae surrounding massive stars. Roberts later expanded this research by studying atomic hydrogen clouds.
Today, scientists use gravitational lensing to map this invisible substance. Giant clusters of galaxies act as natural telescopes, amplifying images of background galaxies. By analyzing the distortions these images undergo, researchers can map how dark matter is distributed throughout the universe. The search for the specific particle responsible for this phenomenon continues through deep underground experiments and advanced telescopes in space and on Earth. High-resolution imagery from the Hubble Space Telescope has become essential for producing these maps. Scientists also conduct searches for dark matter particles in deep underground facilities.
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Sources
- Summary by Intellect · source: The Dark Side of the Universe - Professor Joseph Silk
- Also: The Dark Side of the Universe
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