Dark matter is a mysterious substance that makes up 90% – 99% of the universe’s matter. Its existence was first suggested in the 1930s by astronomers Fritz Zwicky and Sinclair Smith, who observed that galaxies were moving faster than they should be. Further evidence came in the 1970s when scientists analyzed the rotation curves of galaxies. Dark matter is believed to consist of MACHOs and WIMPs, with MACHOs being made up of conventional matter and WIMPs being non-baryonic matter.
Dark matter is a mysterious form of matter that exerts gravitational effects on ordinary matter but cannot be observed by other means. While conventional matter clouds can be detected through their stellar activity, light scattering, radio emissions, and other means, dark matter clouds are invisible to the vast majority of our instruments. Yet they make up 90% – 99% of the matter in the universe.
In the 1930s, astronomers Fritz Zwicky and Sinclair Smith performed an analysis of the velocity of galaxies within the large galaxy clusters Virgo and Coma. They found that all galaxies were moving ten to a hundred times faster than they should have, given estimates based on observed stellar density. Something invisible was generating more gravity.
Although Zwicky and Smith’s initial observations appeared to provide strong evidence for dark matter, not all cosmologists were convinced. Because these galaxy clusters were so far away, it was difficult to accurately measure the independent velocities of each galaxy on a time scale of just a few decades. Stronger evidence came in the 1970s when scientists like Rubin, Freeman and Peebles began analyzing the rotation curves of galaxies. Stars in spiral galaxies, like ours, moved around the galactic core faster than their observable masses would indicate.
All galaxies were eventually found to contain halos of dark matter that extend well beyond the boundaries of the galaxy pointed by a telescope. These halos are said to consist of MACHOs (Massive Astrophysical Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MACHOs are speculated to be primarily black holes and brown dwarfs (burnt stars). They are made up of conventional or baryonic matter – the atoms we are familiar with, only highly compressed. WIMPs are said to be forms of non-baryonic matter, weakly interacting particles that move at relativistic speeds. The most likely candidates for WIMPs are the neutrino and its cousins.
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