We live in an expanding universe. All of the galaxies (vast collections of stars similar to, but outside of, our own Milky Way galaxy) that populate the universe – including our Milky Way – are moving away from each other. How quickly galaxies move away from one another depends on their relative distance. From our viewpoint, the farther away another galaxy is, the faster it moves away from us. This is called the Hubble Law (after the American astronomer Edwin Hubble, who discovered the cosmic expansion in the late 1920s from the 100-inch Hooker Telescope on Mount Wilson near Pasadena, CA).
Although we see galaxies moving away from us in all directions, this does not mean that our galaxy is in the center of some sort of explosion; observers in other galaxies would see the same thing. It only means that the space between all galaxies is growing larger.
Here is a simple example to help you understand the expansion of the universe. Take an uninflated balloon and cover it with dots. Each point represents a galaxy. When you inflate the balloon, the points move away from one another. Notice that no matter which point you choose to represent your location, all points move away from it.
Even though the balloon is stretching uniformly, dots separated by a greater distance move away from each other at a faster speed; the velocity is proportional to the distance. This is the Hubble Law. It happens everywhere on the balloon – and in our universe.
How do we know that galaxies are moving away from ours? It isn't because we see them getting smaller–they don't move that fast. But we observe the frequency of the light from these galaxies (and hence their color) is being shifted to the red end of the spectrum, much as the pitch of a siren atop an ambulance shifts lower as the ambulance moves away from you. The amount of this shift can be measured, and this number is called the "redshift."
The Hubble Space Telescope has contributed to measurements of the expansion of the universe.
Find out more about the expansion of the universe from HubbleSite:
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