Scientists believe that long ago everything in the universe was made out of particles, light, and energy, and it all fit into a single point! Then something happened about 13.8 billion years ago that cause the point to start expanding. As time went on, the particles joined together to form atoms and then larger and larger objects like stars and galaxies. Scientists believe the universe is still expanding and stretching farther and father away from the first point where it all began.
Source: NASA SpacePlace