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Did you know that the matter in your body is billions of years old?

According to most astrophysicists, all the matter found in the universe today -- including the matter in people, plants, animals, the earth, stars, and galaxies -- was created at the very first moment of time, thought to be about 13 billion years ago.

The universe began, scientists believe, with every speck of its energy jammed into a very tiny point. This extremely dense point exploded with unimaginable force, creating matter and propelling it outward to make the billions of galaxies of our vast universe. Astrophysicists dubbed this titanic explosion the Big Bang.

The Big Bang was like no explosion you might witness on earth today. For instance, a hydrogen bomb explosion, whose center registers approximately 100 million degrees Celsius, moves through the air at about 300 meters per second. In contrast, cosmologists believe the Big Bang flung energy in all directions at the speed of light (300,000,000 meters per second, a million times faster than the H-bomb) and estimate that the temperature of the entire universe was 1000 trillion degrees Celsius at just a tiny fraction of a second after the explosion. Even the cores of the hottest stars in today's universe are much cooler than that.

There's another important quality of the Big Bang that makes it unique. While an explosion of a man-made bomb expands through air, the Big Bang did not expand through anything. That's because there was no space to expand through at the beginning of time. Rather, physicists believe the Big Bang created and stretched space itself, expanding the universe.

A Cooling, Expanding Universe

For a brief moment after the Big Bang, the immense heat created conditions unlike any conditions astrophysicists see in the universe today. While planets and stars today are composed of atoms of elements like hydrogen and silicon, scientists believe the universe back then was too hot for anything other than the most fundamental particles -- such as quarks and photons.

But as the universe quickly expanded, the energy of the Big Bang became more and more "diluted" in space, causing the universe to cool. Popping open a beer bottle results in a roughly similar cooling, expanding effect: gas, once confined in the bottle, spreads into the air, and the temperature of the beer drops.

Rapid cooling allowed for matter as we know it to form in the universe, although physicists are still trying to figure out exactly how this happened. About one ten-thousandth of a second after the Big Bang, protons and neutrons formed, and within a few minutes these particles stuck together to form atomic nuclei, mostly hydrogen and helium. Hundreds of thousands of years later, electrons stuck to the nuclei to make complete atoms.

About a billion years after the Big Bang, gravity caused these atoms to gather in huge clouds of gas, forming collections of stars known as galaxies. Gravity is the force that pulls any objects with mass towards one another -- the same force, for example, that causes a ball thrown in the air to fall to the earth.

Where do planets like earth come from? Over billions of years, stars "cook" hydrogen and helium atoms in their hot cores to make heavier elements like carbon and oxygen. Large stars explode over time, blasting these elements into space. This matter then condenses into the stars, planets, and satellites that make up solar systems like our own.

How do we know the Big Bang happened?

Astrophysicists have uncovered a great deal of compelling evidence over the past hundred years to support the Big Bang theory. Among this evidence is the observation that the universe is expanding. By looking at light emitted by distant galaxies, scientists have found that these galaxies are rapidly moving away from our galaxy, the Milky Way. An explosion like the Big Bang, which sent matter flying outward from a point, explains this observation.

Did you know that the static on your television is caused by radiation left over from the Big Bang?

Another critical discovery was the observation of low levels of microwaves throughout space. Astronomers believe these microwaves, whose temperature is about -270 degrees Celsius, are the remnants of the extremely high-temperature radiation produced by the Big Bang.

Interestingly, astronomers can get an idea of how hot the universe used to be by looking at very distant clouds of gas through high-power telescopes. Because light from these clouds can take billions of years to reach our telescopes, we see such bodies as they appeared eons ago. Lo and behold, these ancient clouds of gas seem to be hotter than younger clouds.

Scientists have also been able to uphold the Big Bang theory by measuring the relative amounts of different elements in the universe. They've found that the universe contains about 74 percent hydrogen and 26 percent helium by mass, the two lightest elements. All the other heavier elements -- including elements common on earth, such as carbon and oxygen -- make up just a tiny trace of all matter.

So how does this prove anything about the Big Bang? Scientists have shown, using theoretical calculations, that these abundances could only have been made in a universe that began in a very hot, dense state, and then quickly cooled and expanded. This is exactly the kind of universe that the Big Bang theory predicts.

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