1. Why do astronomers believe that considerable quantities of dark matter must exist in clusters of galaxies? 2. Explain why the dark matter in galaxy clusters cannot be neutral hydrogen. 3. Some...

1. Why do astronomers believe that considerable quantities of dark matter must exist in clusters of galaxies?

2. Explain why the dark matter in galaxy clusters cannot be neutral hydrogen.

3. Some galaxies in the Local Group exhibit blueshifted spectral lines. Why are these blueshifts not violations of the Hubble law?