单选题
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 1 astronomers.
Observations in recent years have changed the basic 2 of how the universe evolved and have indicated how little is known about the major forces and substances that 3 our world.
Astronomers now know that luminous (发光的) matter—stars, planets and hot gas—accounts 4 only about 0.4 percent of the universe. Non-luminous components, such as black holes and intergalactic (星系间的) gas, 5 up 3.6 percent. The rest is either dark matter, about 23 percent, or dark energy, about 73 percent.
Dark matter, sometimes 6 "cold dark matter", has been known for some time. Only recently have researchers come to understand the key role it 7 in the formation of stars, planets and even people.
"We 8 our very existence to dark matter," said physicist Paul Steinhardt and a co-author of a review on dark matter which 9 not long ago in the journal Science.
"Dark matter dominated the structure 10 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 11 a froth (泡沫) of an ocean of dark matter. The dark matter clumps (结成块) and the ordinary matter falls into it. That 12 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 13 . It cannot be seen or detected directly. Astronomers know it is there because of its 14 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 15 power that astronomers say is causing the galaxies in the universe to separate at a faster and faster speed.