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The actual number:

How far apart are galaxies on average?

An attempt to visualize such a thing:

If galaxies were the size of peas, how many would be in a cubic meter?

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The simple answer is that the average galaxy spacing is around a few megaparsecs, while the biggest galaxies are around 0.1 megaparsecs in size. So the average spacing is somewhere in the range of 10 - 100 times the size of the biggest galaxies. The peas I had for lunch today were (at a guess - I didn't measure them!) 5mm in diameter so the interpea spacing would be 5 - 50cm, or between 8,000 and 8 per cubic metre.

But this is a very misleading statistic. Galaxies are not distributed uniformly, but instead are grouped into clusters, which are themselves grouped into superclusters. Also galaxies vary enormously in size, with dwarf galaxies around a thousand times smaller than the biggest galaxies.

I would resist the temptation to assign any significance to my figures above. However there is a take home message i.e. galaxies are much, much, much closer relative to their size than stars are. That's why galaxy collisions are quite frequent while stellar collisions are rare to the point of non-existance.

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+1 for including a note on significance of statements! I was thinking of a homogeneity scale, so (super)clusters are an important detail. –  user12345 Mar 31 '13 at 14:25
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