D．was pleased with what they had done

　　20．What would be the best title for the passage?

　　A．Adopted Sisters Are Reunited

　　B．Adopted Sisters Live Happily in the US

　　C．Suspicion Turned into Reality

　　D．The Stories of Two FamiliesB

　　The earth，moon，sun and all visible stars in the sky make up less than one percent of the universe.Almost all the rest is dark matter and dark energy，unknown forces that puzzle astronomers.

　　Observations in recent years have changed the basic understanding of how the universe evolved and have indicated how little is known about the major forces and substances that shaped our world.

　　Astronomers now know that luminous(发光的) matter—stars，planets and hot gas...accounts for only about 0.4 percent of the universe.Non­luminous components，such as black holes and intergalactic(星系间的) gas，make up 3.6 percent.The rest is dark matter，about 23 percent，and dark energy，about 73 percent.

　　Dark matter，sometimes called “cold dark matter”，has been known for some time.Only recently have researchers come to understand the key role it played in the formation of stars，planets and even people.

　　“We owe our very existence to dark matter，” said physicist Paul Steinhardt.“Dark matter dominated the structure formation in the early universe，” Steinhardt said.“For the first few billion years dark matter contained most of the mass of the universe.You can think of ordinary matter as a froth(泡沫状物) of an ocean of dark matter.The dark matter dumps and the ordinary matter falls into it.That led to the formation of the stars and galaxies(星系)．Without dark matter，there would be virtually no structure in the universe.”

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