The Apparent Movement of Galaxies
The expansion of space causes galaxies to appear to be moving apart from each other. Although it may seem that the galaxies themselves are moving through space, in reality it is the space between the galaxies that is growing.
How does Hubble's Law explain the receding galaxies?
Imagine space as being represented by a rubber band. If one stretches the rubber band the galaxies will move apart. From the perspective of each galaxy, it will appear as though all of the other galaxies are moving away from it, while it remains stationary. The closer two galaxies are to each other, the more slowly they appear to move apart. This is in accordance with Hubble's Law, which states that the redshift in light coming from distant galaxies is linearly proportional to its distance from Earth.
Models of Expansion
The Universe as a Loaf of Bread
Another way to think about expansion is to imagine the Universe is a loaf of raisin bread. When baked in the oven, the bread expands, but the raisins do not. The bread represents the space in the Universe, and the raisins represent galaxies and other astronomical objects . While the bread itself undergoes a large change in structure, the raisins themselves stay the same.
The Universe as Latticework
Cubic Space Division by M.C. Escher
Another helpful way to visualize the Universe is as a huge latticework, like the one in M.C. Escher's sketch.
In this analogy, each cube is a galaxy (or cluster of galaxies), and the connecting rods represent the space between these galaxies. When the Universe expands, imagine that every rod connecting the cubes grows longer at an even pace. Every single cube gets further and further away from every other cube, but the size of the cubes themselves stays the same. The cubes are not moving along the latticework but are being carried by the expansion of the lattice itself.
The Big Bang, then, is not an explosion in space, but rather, an explosion of space!
We are Not at the Center
If you view the expansion of space from the Earth's perspective, it appears that everything is expanding away from us. However, no matter where you are in the universe, that location will appear to be the center of expansion, too. The universe expands from every point!