This image represents an expanding Universe. The arrows show that the movement is outward from the center. The spirals represent galaxies.
STsci
Hubble Flow: The Expanding Universe
In the 1920's the famous American astronomer Edwin Hubble made an amazing discovery. He found that, no matter which direction he looked into space, distant galaxies appeared to be moving away from us. The farther away the galaxy is from our galaxy the faster it moves away from us. Hubble was observing the expansion of the Universe.
What does it mean that the Universe is expanding? Imagine baking a loaf of raisin bread. As the bread rises it also expands. All of the raisins move farther apart from one another. Every single raisin would see all of the others moving away from it. All of the galaxies in the universe are like the raisins in the bread.
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