Briefly explain how dark matter affects the expansion rate of the universe and the significance of the term critical density in the absence of any mysterious repulsive force

Question

What will be an ideal response?

Answer 1

Dark matter slows the expansion of the universe, because gravity is an attractive force. In the absence of any mysterious repulsive force (or dark energy), a universe with the average mass density equal to the critical density is the densest universe that will expand forever. A denser universe will eventually reverse its expansion and contract; a less dense universe will expand forever.