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 explosion set the Universe into motion and this motion continues today. Scientists are not yet sure if the motion will stop, change direction, or keep going forever.