What is one of the big puzzles about the properties and behavior of large clusters of galaxies?
Their gravity seems to show some sort of hidden mass, which we cannot detect directly. All we can see is the gravity.
That hidden mass, whatever it is, is called dark matter (https://en.wikipedia.org/wiki/Dark_matter). This dark matter is believed to consist of more mass than ordinaru matter, by more than 5 to 1, yet it is spread so thinly that we do not see its gravity on an interplanetary or even interstellar distance scale. We see its gravity by looking at galactic scale motion. Our galaxy rotates so fast is should fly apart eccept for the mysterious extra gravity of datk matter holding it in place, and clusters of galaxies are similarly more strongly bound than they "should be".
No one really knows what this dark matter is made of; if we did we would count its mass with ordinary matter and we could predict how strongly galaxies are really held together. Weakly interacting massive particles, or WIMPs, are a leading candidate (https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles). A WIMP is a hypothesized type of neutral fundamental particle that interacts through gravity and the weak nuclear force. So far we have not seen WIMPs, and their detection is a major goal of high-energy particle physics today.