One way to think of the universe is in terms of a vast three-dimensional web with vast empty spaces, intergalactic voids, nested in-between filaments consisting of galaxies. There are clusters of galaxies at the nodes of these filaments.
Cosmologists and astrophysicists are not sure why the universe is organised this way. There are multiple competing models of how matter, energy, dark matter and dark energy could have interacted to set up this structure. Every time new data comes to light, those models are reviewed and modified.
A cluster of galaxies can contain thousands of separate galaxies forming a sort of wall of
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