Until recently most astronomers believed that the space between the galaxies in our universe was a near- perfect vacuum. This orthodox view of the universe is now being challenged by astronomers who believe that a (5) heavy "rain" of gas is falling into many galaxies from the supposedly empty space around them. The gas apparently condenses into a collection of small stars, each a little larger than the planet Jupiter. These stars vastly outnumber the other stars in a given galaxy. The (10) amount of "intergalactic rainfall" into some of these galaxies has been enough to double their mass in the time since they formed. Scientists have begun to suspect that this intergalactic gas is probably a mixture of gases left over from the "big bang" when the galaxies were (15) formed and gas was forced out of galaxies by supernova explosions.
It is well known that when gas is cooled at a constant pressure its volume decreases. Thus, the physicist Fabian reasoned that as intergalactic gas cools, the cooler gas (20) shrinks inward toward the center of the galaxy. Mean- while its place is taken by hotter intergalactic gas from farther out on the edge of the galaxy, which cools as it is compressed and flows into the galaxy. The net result is a continuous flow of gas, starting as hot gases in inter- (25) galactic space and ending as a drizzle of cool gas called a "cooling flow," falling into the central galaxy.
1. The primary purpose of the passage is to
(A) illustrate a hypothesis about the origin of galaxies
(B) provide evidence to dispute an accepted theory about the evolution of galaxies
(C) summarize the state of and prospects for research in intergalactic astronomy
(D) report new data on the origins of intergalactic gas
(E) reconcile opposing views on the formation of intergalactic gas