Before the universe as we now know it existed, there was no space or time. The Big Bang and its
associated theories try to explain or describe the moment of change from nothingness and no time to the
existence of the universe filled with space and marked by time. Many physicists describe this event as an
explosion, or flash, hence the name Big Bang. The Big Bang is a process of expansion in our universe that
is still active today.

The universe flashed into existence (according to the Big Bang theory) from a very small agglomeration of
matter of extremely high density and temperatures. As a dense, hot globule of gas, containing nothing but
hydrogen and a small amount of helium, it began expanding rapidly outward. There were no stars or planets.
The first stars probably formed when the universe was about 200 million years old. Our Sun was formed 4.5
billion years ago, and through telescopes we can now see stars forming out of compressed pockets of
hydrogen in outer space.

In 1992 instruments aboard the Cosmic Background Explorer (COBE) satellite, launched in 1989, showed
that 99.97% of the radiant energy of the universe was released within the first year of the Big Bang event.
This evidence seems to confirm the Big Bang theory. In March 1995 astronomers found more supporting
evidence for the big bang when they concluded that data obtained from the space shuttle's Astro 2
observatory showed that helium was widespread in the early universe. The Big Bang theory holds that
hydrogen and helium were the first elements created when the universe was formed.

In one of the most important cosmological discoveries in years, NASA scientists have captured the most
precise image of the universe, shedding light on its origins, age, and providing further evidence for the long-
standing Big Bang and inflation theories. In Feb. 2003, a joint NASA–Princeton University satellite, the
Wilkinson Microwave Anisotropic Probe (WMAP), produced a high-resolution map that captured the oldest
light in the universe. This ancient light, called the cosmic microwave background, is the cooled remnant of
the hot explosion known as the Big Bang. “The cosmic microwave light is a fossil,” explained David
Wilkinson, after whom the probe was named, “Just as we can study dinosaur bones and reconstruct their
lives of millions of years ago, we can probe this ancient light and reconstruct the universe as it was.”
(Wilkinson died just before WMAP's amazing findings were published.)

The age of the universe has now been accurately determined—with just a 1% margin of error—as 13.7
billion years old (previous estimates ranged between 8–20 billion years old). The birth of stars has been
pinpointed to just 200 million years after the Big Bang, a surprise to most scientists (predictions had ranged
from 500 million to 1 billion years after the cosmos formed). The WMAP image also revealed the contents
of the universe: only 4% is made up of atoms, or the physical universe as we know it. The remainder is
made up of poorly understood substances: dark energy (73%) and dark matter (23%). These findings are
consistent with the Big Bang and inflation theories, which assert that the universe materialized in a “big
bang” and immediately began cooling and expanding. “I think every astronomer will remember where they
were when they heard these results,” said John Bahcall, a Princeton University astrophysicist. “I certainly
will. This announcement represents a rite of passage for cosmology from speculation to precision science.”

13.7 billion years: Age of the universe
200 million years: Interval betwen the Big Bang and the appearance of the first stars
4%: Proportion of the universe that is ordinary matter
23%: Proportion that is dark matter
73%: Proportion that is dark energy