Dark Matter and Dark Energy: Practice Problem Solutions

1. About 5% ordinary matter, 27% dark matter, 68% dark energy.

2. The galaxies moved faster than the cluster's visible mass could hold, so there must be extra, unseen mass.

3. Stars far from the center move about as fast as stars closer in, instead of more slowly. That implies a large amount of unseen mass in a halo.

4. After two clusters collided, the mass (found by lensing) passed through with the galaxies, while the hot gas stayed in the middle. This supports mass that is separate from ordinary matter.

5. Astronomers measured distant Type Ia supernovae and found them fainter than expected, showing that the expansion of the universe is speeding up.

6. WIMPs or axions (others include sterile neutrinos and primordial black holes).

7. Dark matter and dark energy make up about 95% of the model's contents, but we only know their effects (gravity and speeding up), not what they are.