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第6部分：完形填空(第51～65题，每题1分，共1 5分) 阅读下面的短文，文中有15处空白，每处空白给出了4个选项，请根据短文的内容从4个选项中选择1个最佳答案，涂在答题卡相应的位置上。 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)matterstars，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. | 
