The Big Bang
Everywhere we look in the distant regions of space, galaxies appear to be rushing away from us at faster and faster speeds. But the galaxies are not really moving. The space between the galaxies is stretching.
The Big Bang wasn't a massive explosion with matter and energy flying off into a previously empty space. Instead, imagine the entire universe as a giant expanding bubble with the galaxies floating on its surface. The growing surface moves the galaxies apart, but the individual galaxies, stars and planets stay the same size.