*a.	It emits no radiation of any wavelength that we have been able to detect.
b.	It blocks out the light of stars in a galaxy.
c.	It contains large amounts of dark-colored dust.
d.	It emits no visible light.

a.	The luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter.
*b.	The unseen dark matter represents much more mass and extends much further from the galactic center than the luminous matter in the Milky Way.
c.	Luminous matter emits white light, much like the light reflected from icebergs.
d.	Black holes are much more interesting than ordinary stars that give off light.

a.	Studying the X-ray emission from hot gas inside the clusters.
b.	Measuring the speeds of galaxies orbiting the cluster's center.
*c.	Measuring the temperatures of stars in the halos of the galaxies.
d.	Observing how the clusters bend light as gravitational lenses.

*a.	It bends or distorts the light coming from galaxies located behind it.
b.	The overall shape of the cluster is that of a lens.
c.	It is an unusually large cluster that has a lot of gravity.
d.	It magnifies the effects of gravity that we see in the cluster.

a.	Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.
b.	Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.
c.	Dark matter is present between galaxies in clusters, but not within individual galaxies.
*d.	Dark matter is the dominant form of mass in both clusters and in individual galaxies.

a.	It is made of subatomic particles that scientists call WIMPs.
*b.	It consists of atoms or ions with nuclei made from protons and neutrons.
c.	It emits a great deal of light.
d.	It can attract other matter through the force of gravity.

a.	Dim, low-mass stars.
b.	Brown dwarfs.
c.	Planet-sized objects that do not orbit a star.
*d.	Neutrinos.

a.	They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily.
b.	They interact with other matter ONLY via the weak force and not via gravity or any other force.
*c.	They interact with other matter via the weak force but do not interact with the electromagnetic force and hence cannot emit light.
d.	The light that they emit is so weak that it is undetectable to our telescopes.

*a.	Aside from some suspicions that it dark matter is made largely of undiscovered subatomic particles, we really we don't have a very good idea of what it is.
b.	Dark matter consists 90% of MACHOs and 10% of WIMPs.
c.	We are certain that dark matter is made mostly of WIMPs.
d.	Dark matter probably does not really exist.

a.	Any individual cluster of galaxies is considered a large-scale structure.
*b.	The overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe.
c.	The overall shape of the observable universe.
d.	Any individual galaxy is considered a large-scale structure.

a.	It is the actual average density of the universe.
b.	It is the density necessary in a particular region of space in order for a supercluster to form in that region.
c.	It is the density of dark matter in the universe.
*d.	The average density the universe would need in order for its gravity to ultimately win out over the current rate of expansion (if it weren't accelerating).

*a.	The expansion of the universe seems to be getting even faster.
b.	The expansion of the universe seems to be slowing down.
c.	The expansion of the universe is only true for us, as observers in the Milky Way.
d.	The expansion of the universe seems to be constant.

a.	The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
b.	The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.
c.	Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster.
*d.	Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.

*a.	They travel at speeds close to the speed of light.
b.	They tend to orbit at large distances from the galactic center.
c.	They can neither emit nor absorb light.
d.	They are subatomic particles.

a.	Space IS expanding within clusters of galaxies, which is why clusters tend to grow in size with time.
*	Their gravity is strong enough to hold them together even while the universe as a whole expands.
c.	The universe is not old enough yet for these objects to have begun their expansion.
d.	Expansion of the universe affects only empty space, not space in which matter is present.

*a.	Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
b.	Galaxies and clusters and the structures in which they are embedded form from tiny density enhancements that were present in the early universe.
c.	Voids began their existence as regions in the universe with a slightly lower density than the rest of the universe.
d.	Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members.

*a.	We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
b.	The acceleration is very important in the cosmos today, but we think the acceleration will eventually slow down.
c.	We are quite confident that we understand the cause of the acceleration: the cause is dark energy.
d.	The acceleration was something a few astronomers thought they saw, but we no longer think it is real and assume that the original claims were based on errors in interpreting the data.