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.”

　　The nature of dark matter is unknown.It cannot be seen or detected directly.Astronomers know it is there because of its effects on celestial(天上的) objects that can be seen and measured.But the most dominating force of all 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.

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