What is one of the big puzzles about the properties and behavior of large clusters of galaxies?

Question

Ivy Allen · Accepted Answer

Their gravity seems to show some sort of hidden mass, which we cannot detect directly. All we can see is the gravity.

Whatever that hidden mass is, it's known as dark matter (https://tutor.hix.ai). It is thought to contain more mass than ordinary matter—by a factor of more than 5—but it is dispersed so thinly that we are unable to detect its gravity on an interplanetary or even interstellar distance scale. Instead, we detect it through the motion of galaxies within clusters, which is similarly more strongly bound than "should be" due to the mysterious extra gravity of dark matter. A major goal of modern high-energy particle physics is to detect weakly interacting massive particles, or WIMPs (https://tutor.hix.ai), a type of hypothetical neutral fundamental particle that interacts through gravity and the weak nuclear force. Although the composition of this dark matter is unknown, if we did know, we could count its mass with ordinary matter and predict the true gravitational force holding galaxies together.

Leah Abbate · Answer

undefined One of the big puzzles about the properties and behavior of large clusters of galaxies is the discrepancy between the observed mass and the mass calculated from the visible matter within the cluster. This suggests the presence of dark matter, which constitutes a significant portion of the cluster's mass but does not emit or interact with electromagnetic radiation, making it difficult to detect directly.