Our brains struggle to comprehend how big the universe is because everything here on Earth, and even the Earth itself, is very small when compared to the immense scale of the universe. So let's think about it a different way, using something we see and interact with every day… light.
While we imagine light to be instantaneous, photons of light actually take time to travel from one side of the room to the other.
In the time it took you to read this far, a photon of light leaving the Sun has travelled about 10 million kilometres – equivalent to travelling around the Earth 250 times.
Light that leaves our second nearest star, Proxima Centauri, takes just over four years to reach Earth and so we can define it as four light years away.
As such, if you were to look at Proxima Centauri, you would not be seeing the star as it is right now, but how it 'was' 4 years ago!
We see all things in the universe as they were in the past, whether they're on the other side of the room or the other side of the galaxy.
To take this concept further, the nearest large galaxy to us is Andromeda which is so big and close that you can see it in the night sky with your naked eye.
What you're really seeing is 1,000's of billions of stars in a configuration similar to our Milky Way. However, all of those stars are about 2.5 million light years away, which means you're seeing Andromeda as it was 2.5 million years ago.
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