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Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.
— The Hitchhiker's Guide to the Galaxy
When we say "space is big", it's partly meant as a correction to images like this:
Wikimedia source.
where, for simplicity, celestial objects are shown in unrealistically close proximity—like Jupiter is just down the hall from Mars.
As we all know, the planets of the solar system are actually really far apart, even relative to their huge size. In Sweden's 1:20,000,000 to-scale solar system model, Earth and Venus are just ~2 feet in diameter, yet over a mile apart. In other terms, they're several thousand "Earth-widths" apart:
Horizontal distance not to scale, wouldn't fit on screen.
And we all know that's just the start of it: the stars are incredibly far apart. Finland also has a to-scale model of the solar system, but it's 150x more condensed than Sweden's at 1:3,000,000,000.[1] Its model of our sun, Sol, is just ~1.5 feet in diameter.
Its model of the nearest star, Proxima Centauri, is in Australia.[2]
Horizontal distance obviously not to scale.
That's even crazier than Earth-Venus, especially considering a "Sun-width" is ~865,000 miles.
So if we keep going bigger, what happens then? What about galaxies?
That image is to scale. If the Milky Way were a typical US suburban home, Andromeda would be a McMansion 3 blocks down the road.[3]
And while many people know that Earth's Moon is not terribly close to Earth:[4]
Again, image here is to scale.
How many know that the Milky Way's "moon", the Large Magellanic Cloud, is about as close as Earth's Moon is often misleadingly depicted?
To scale.
There are some interesting implications here; in particular, if your star-faring civilization can colonize the entire Milky Way, you can probably make it over to Andromeda without much more difficulty.[5]
And indeed, you wouldn't stop there either. The next