Expansion of the Universe

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Edwin Hubble
When we look at galaxies which are far, far away, we see they are all moving away from us. And, the further away the galaxy is, the faster it is moving. This is happening because the space between the galaxies is stretching.
No matter where you are in the Universe everything appears to move away from you. You can do an experiment with a balloon to help you understand why this happens. This means the entire Universe is expanding. This is a key piece of evidence for the Big Bang theory. 

In 1929, Edwin Hubble measured the speed at which galaxies are moving away from us. He looked at the light from lots of galaxies and measured the amount of redshift in the light. Hubble used his results to make a discovery. The speed of the galaxies was faster, the further away they were. This relationship between speed and distance is now called Hubble's Law

The rate at which the Universe is expanding is called the Hubble constant. It is very hard to get an accurate and precise number for the Hubble constant. Scientists have used different way to work it out, but not all the answers agree. Recent answers suggest the Universe is expanding at a faster rate than scientists expected it to. Astronomers do not yet know why this would be happening. One idea is there could be some kind of force which is pulling galaxies apart. Astronomers have named this force, dark energy.