Observations in recent years have changed the basic understanding
of how the universe evolved and have emphasized for astronomers how
little is known about the major forces and substances that shaped
Astronomers now know that luminous matter stars, planets and
hot gas account for only about 0.4 percent of the universe.
Nonluminous components, such as black holes and intergalactic gas,
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 time. Only recently have researchers come to understand the
pivotal role it played in the formation of stars, planets and even
"We owe our very existence to dark matter," said Paul Steinhardt,
a physicist at Princeton University and a co-author of a review on
dark matter appearing this week in the journal Science.
Steinhardt said it is believed that following the Big Bang, the
theoretical beginning 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 fundamental
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 most of the mass of the universe. You can think of
ordinary matter as a froth on an ocean of dark matter. The dark
matter clumps and the ordinary matter falls into it. That led to the
formation of the stars and galaxies."
Without dark matter, "there would be virtually no structures in
the universe," he said.
The nature of dark matter is unknown. It cannot be seen or
detected directly. Astronomers know it is there because of its
effect on celestial objects than 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. It is the force that is causing the universe to expand at an
Robert P. Kirshner, an astronomer at the Harvard-Smithsonian
Center for Astrophysics, said the presence of dark energy was proved
only five years ago when astronomers studying very distant exploding
stars discovered they were moving away at a constant acceleration.
It was a stunning discovery that has since been proved by other
Kirshner said it is clear now that dark matter and dark energy
engaged in a gravitational tug of war that, eventually, dark energy
Following the Big Bang some 14 billion years ago, matter in the
universe streaked outward. It formed galaxies, thinned out and then
began to slow down.
"Dark matter was trying to slow things down and dark energy was
trying to speed it up," said Kirshner, the author of a review
article on dark energy in Science.
"We think dark matter was winning for the first seven billion
years, but then universe went from slowing down to speeding up. ...
Dark energy took over."
Kirshner said astronomers do not really understand dark energy.
Albert Einstein first proposed a form of the idea, but discarded it
later. Now, researchers know it exists, but its exact form and
nature are mysterious, although it is thought to be related to
"What this is pointing to is a deep mystery at the heart of
physics," said Kirshner. "We don't understand gravity in the same
way we understand other forces."
He said there are virtually no experiments on Earth that would
explore the nature of dark energy. It can only be studied across
vast stellar distances by observing the motion of objects extremely
far away, a skill that has been possible only in recent decades with
the development of very powerful telescopes.
"Dark energy will cause the universe to expanded faster and
faster and eventually, over time, we will see less and less of it,"
Kirshner said. Over millions of years, familiar stars and nearby
galaxies will disappear from view and the sky, now choked with
stars, will slowly darken.
"The piece of the universe that we can see will get lonelier and
lonelier," he said.
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