One of the big puzzles about the properties and behavior of large clusters of galaxies is that
A) each cluster appears to consist of the same type of galaxy, some made up totally of spiral galaxies while others contain only ellipticals.
B) they appear to be spread uniformly throughout space in all directions, which is difficult to explain with the Big Bang theory.
C) there appears to be insufficient mass in the luminous matter (stars, and so on) to hold the cluster together gravitationally.
D) they appear not to take part in the general expansion of the universe, in contrast to single separate galaxies, probably because they are gravitationally bound to one another.

Question

Quizplus · Accepted Answer

Clusters of galaxies exhibit gravitational effects that suggest there is more mass present than what we can see directly through luminous matter like stars and galaxies. This discrepancy has led to the hypothesis of dark matter, which does not emit light but exerts gravitational forces.

One of the Big Puzzles About the Properties and Behavior