One of the most significant open questions in physics today is the nature of dark matter. While we have not directly detected dark matter particles, we have been able to infer their existence from their gravitational effects on visible matter and radiation—such as the unexpected behavior of galaxy rotation curves, gravitational lensing patterns, and anisotropies in the cosmic microwave background (CMB). From these observations, it is estimated that dark matter accounts for about 85% of all matter in the universe
One of the key ways we know dark matter exists is through rotation curves. Kepler’s third law states that as the radius of an orbit increases, the velocity with which an object orbits should decrease. However, we have observed for a long time that the rotation curves of galaxies are flat. In other words, the velocity of all objects in the galaxy is relatively constant, regardless of the radius of the orbit. In order for both Kepler’s third law and our observations to be true, there must be some unseen mass in every galaxy that would provide enough force to speed the orbits up such that they would match with our observations.
As of right now, there are three main candidates for what kind of particle dark matter could be. First, MACHOs, which stands for Massive Compact Halo Object. This includes black holes, neutron stars, brown dwarfs, rogue planets, and faint stars. These are objects with a very large mass that just don’t give off enough light for us to see them. However, MACHOs are not looking promising at the moment as a particle candidate because research has shown that they don’t make up enough mass to be the 85% of the universe that is dark matter. Second, WIMPs, which are Weakly Interacting Massive Particles. These are theoretical particles that have been created to match the conditions for dark matter. That is, it has to be cold, or slow moving, it has to interact with the weak force, it has to be relativistic, and it has to be massive, leading to the classification of WIMP. Lastly, axions, another theoretical particle. Axions, however, have much smaller mass ranges, meaning that axions would have to be incredibly abundant. The idea of the axion was originally created to solve the strong CP problem, and was later theorized to be dark matter. Axions are the most popular theory among the dark matter particle candidates today.
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