How do we know that galaxy clusters contain a significant amount of mass in the form of hot gas?

Galaxy clusters are known to contain a significant amount of mass in the form of hot gas, primarily because of the way this gas emits radiation. When we use X-ray telescopes to observe galaxy clusters, we can detect the high-energy X-rays that are produced by the hot, ionized gas that fills the space within the cluster. This gas, which can exceed temperatures of millions of degrees, interacts with the gravitational field of the cluster, allowing us to estimate the amount of gas present.

The detection of X-ray emission is particularly effective because hot gas emits X-rays through a process called thermal bremsstrahlung, or free-free emission, where electrons are accelerated by the electric fields of ions and emit radiation. This emission provides direct evidence of the gas's presence and allows astronomers to calculate the temperature and density of the gas, leading to estimates of its total mass within the cluster.

Although other methods of observation, like using optical telescopes, could identify galaxies within the cluster or other visible structures, they do not provide direct information about the hot gas that contains a significant mass component. Similarly, gravitational waves are primarily associated with cataclysmic events such as mergers and are not a means to detect gas within the cluster. Therefore, the effective use