1. We know that there must be dark matter in the universe because
(a) We see dark dust clouds.
(b) We see that the universe is expanding.
(c) We see that stars far from the galactic center are moving faster than can be explained by visible matter.
(d) We see that the expansion of the universe is accelerating.
2. Acceleration of the universe's expansion rate is due to
(a) The repulsive effect of dark energy.
(b) The attractive effect of dark matter.
(c) The attractive effect of gravity.
(d) The thermal expansion of stellar cores