Over 70 years ago, Edwin Hubble announced his findings that the universe appeared to be expanding in all directions. Since then, scientists have been trying to measure the rate of expansion. Interestingly, the rate of expansion is not the same everywhere in the universe. The rate galaxies move away from each other is proportional to distance (the farther away the galaxy, the faster it seems to be moving awa). Therefore, the rate of expansion of the universe, known as Hubble's constant, is in units of velocity per distance. So, for example, if an object is twice as far away, it will appear to be moving away at twice the velocity. Reported values of Hubble's constant seem to vary between 50 to 100 km/s/megaparsec (a megaparsec is about 3.3 million light years). A few years ago, a study using the Hubble Telescope came to a value of 70 km/s/megaparsec.

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The current date for this answer is December 5th 2002.

Well, when we say the universe is expanding, we mean that space itself is getting bigger and pushing the galaxies apart. Imagine putting dots on a balloon and then blowing it up. As the balloon expands, the dots move apart. And the farther apart the dots are, the faster they move away from each other. So we measure the expansion of the universe by saying that, if a galaxy is so far away, it is moving away from us so fast. This measurement is called the Hubble parameter, and it is roughly that a galaxy is moving away from us about 70 kilometers/second for every megapersec that the galaxy is distant from us. A megapersec is about 3 million light years (a light year is the distance light travels in a year, and light takes only about 8 minutes to reach the sun!). If I've done my conversion factors correctly, this means that the universe expands by about 0.0000000001% every year.