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 1 that puzzle astronomers.
Observations in recent years have changed the basic understanding of how the universe evolved and have emphasized for astronomers how little is known 2 the major forces and substances that shaped our world.
Astronomers now know that luminous matter-stars, planets and hot gas— 3 only about 0.4 percent of the universe. Non-luminous components, such as black holes and intergalacticgas, make up 3.6 percent. The rest is either dark matter, about 23 percent, or dark energy, about 73 percent.
Dark matter, sometimes called "cold dark matter", has been known for some 4 . Only recently have researchers come to understand the pivotal role it 5 in the formation of stars, planets and even people.
"We owe our very existence to dark matter," said physicist Paul Steinhardt and a co-author of a review on dark matter which appeared not long 6 in the journal Science.
Steinhardt said it is believed that following the Big Bang, the theoretical 7 of the universe, dark matter caused particles to clump together. That set up the gravitation processes that led to the formation of stars and galaxies. Those stars, in turn, created the basic chemicals, such as carbon and iron, that were 8 to the evolution of life.
"Dark matter dominated the formation of structure in the early universe," Steinhardt said. "For the first few billion years dark matter contained 9 of the mass of the universe. You can think of ordinary matter as a froth of an Ocean of dark matter. The dark matter clumps and the ordinary matter falls into it. That led to the 10 of the stars and galaxies."
Without dark matter, "there 11 be virtually no structures in the universe."
The nature of dark matter is 12 . It cannot be seen or detected 13 . Astronomers know it is there because of its effect on celestial objects than can be seen and measured.
But the most dominating force of 14 in the universe is called dark energy. A recently proven power that astronomers say is causing the galaxies in the universe to separate at a faster and faster speed.
One scientist said it is clear now 15 dark matter and dark energy engaged in a gravitational tug of war that, eventually, dark energy won.