Here's why:
* The Big Bang: The universe began with a massive expansion known as the Big Bang.
* Spacetime Expansion: The expansion isn't just galaxies moving through space, but rather space itself expanding. Think of it like a balloon with dots drawn on it. As you inflate the balloon, the dots move further apart, even though they are not moving on the balloon's surface.
* Hubble's Law: Astronomer Edwin Hubble discovered a relationship between the distance to a galaxy and its recessional velocity (speed moving away from us). This relationship is known as Hubble's Law. The farther a galaxy is, the faster it appears to be moving away.
Important Note: While almost all galaxies are moving away from us, it's not a matter of them being pushed away. The space between them is simply expanding.
