The Universe is a huge wide-open space that holds everything from the smallest particle to the biggest Galaxy. No one knows just how big the Universe is.

Astronomers try to measure it all the time. They use a special instrument called a spectroscope to tell whether an object is moving away from Earth or toward Earth. Based on the information from this instrument, scientists have learned that the Universe is still growing outward in every direction.

Scientists believe that over 15 billion years ago, a powerful explosion called the Big Bang happened. This powerful cosmic explosion set the Universe into motion, thrusting matter in all directions. This motion even continues today as the Universe is expanding. Scientists are not yet sure if the motion will stop, change direction, or keep going forever.

Before the Big Bang explosion happended the entirety of our universe was compressed into the confines of an atomic nucleus. Known as a singularity, this is the moment before creation when space and time did not exist. According to cosmological models that explain our universe, an overwhelming explosion, trillions of degrees in temperature on any measurement scale, that was infinitely dense, created not only fundamental subatomic particles and thus matter and energy but space and time itself.

Depending on the average density of matter and energy in the universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "Big Crunch".

Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for the whole of eternity. Other ideas of the fate of our universe include the Big Rip, the Big Freeze, and Heat death of the universe theory.