Scientists believe that the Universe began in a big explosion called the Big Bang. It started off as one tiny point, too small to see, but has been getting bigger ever since.
We don’t really know anything about the first moments of the Universe, but very early on, the Universe grew very quickly in a very short time. This is called “inflation”. When inflation ended, the Universe was 1,000,000,000,000,000,000,000,000,000 times bigger than it had been.
The Universe carried on growing much more slowly, and started to cool down. Light was made very early on, followed by dust and gas. In some places there was more of this dust and gas than in others. This was pulled in by gravity to make galaxies.
The earliest of these galaxies can be seen as faint blue dots in the Hubble Deep Field. This image covers a tiny patch of sky - only 1/30 the size of the full Moon - which seemed to be completely empty before the Hubble Space Telescope took this picture.
The Size of the Universe
We don’t know how big the Universe really is because we can’t see all the way to the edge of it. This is because light takes a long time to travel from very far off galaxies. The Universe is only 15,000,000,000 years old, which sounds like a long time but isn’t long enough for the light from some very far off galaxies to have reached us yet.
Looking at the light from distant stars is like looking back in time. If you look up at a star, the light that you see left that star millions of years ago. You are seeing the star as it looked then, not as it looks now. You would have to wait another few million years for the light that is leaving the star today to travel all the way through space to Earth before you could see how the star looks now.
How the Universe will End
We don’t really know how the Universe is going to end. It might stop growing and fall back in again in a “Big Crunch” or it might keep growing forever. At the moment it looks like the Universe is growing faster and faster, so it will probably keep growing forever.
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