Which single major problem perhaps puzzles astronomers the MOST as they attempt to interpret the properties and behavior of clusters of galaxies?

Question

Quizplus · Accepted Answer

The missing-mass problem, also known as the dark matter problem, is a major unsolved issue in astronomy. The observed visible mass of galaxies in clusters is not sufficient to explain their gravitational stability, hence a large amount of invisible dark matter is required. This has puzzled astronomers for decades, as they try to understand the nature and properties of dark matter.

Which Single Major Problem Perhaps Puzzles Astronomers the MOST as They