单项选择题

Dark Forces Dominate Universe
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 ______(51)astronomers．
Observations in recent years have changed the basic______ (52)of how the universe evolved and have indicated how little is known about the major forces and substances that______ (53) our world Astronomers now know that luminous(发光的)matter—stars，planets and hot gas----Accounts ______ (54) only about 0,4 percent of the universe．Non-luminous components，such as black holes and intergalactic(星系间的)gas，______ (55)up 3.6 percent The rest is either dark matter , about 23 percent，or dark energy，about 73 percent．
Dark matter．Sometimes ______ (56)“cold dark matter，”has been known for some time．Only recently have researchers come to understand the key role it ______ (57)formation of stars，planets and even people．
We ______ (58) our very existence to dark matter,” said physicist Paul Steinhardt and a co-author of a review on dark matter which ______ (59)not long ago in the journal Science． “Dark matter dominated the structure (60)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 (61) a froth(泡沫)of an ocean of dark matter．The darkmatter dumps(结成块)and the ordinary matter falls into it．That (62) to the formation of the stars and galaxies(星系)．”Without dark matter ,“ there would be virtually no structures in the universe
The nature of dark matter is ______ (63)It cannot be seen or detected directly Astronomers know it is there because of its______ (64)on celestial(天体的)objects that can be seen and measured． But the most dominating force of all in the universe is called dark energy, a recently ______ (65)power that astronomers say is causing the galaxies in the universe to separate at a faster and faster speed.

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