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The CMB cold spot (if it exists) may imply a supervoid of approximately 1 billion light years across. If the diameter of the Observable Universe is 46 billion light years this is about 2% the diameter.

Could anything bigger than this exist without us knowing? For example, could a void or super-super-cluster exist 20% the diameter of the Universe without us realising yet?

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Galaxy filaments can be comparable in size to what you're talking about. Some of the largest ones exceed 1 billion light-years in length, including

Filaments are expected to form from density fluctuations in the early universe, and subsequent interactions with dark matter.

Large quasar groups are claimed to be even larger, although there are some disputes as to whether some of these are true structures:

Observations suggest that large quasar groups are the progenitors of galaxy superclusters (as they occur at redshifts of $z\simeq1$). They do seem to cluster in similar densities as supercluster members, making that evolutionary path all the more appealing.

It's often argued that structures on this scale - say, 1 billion light-years - violate, or come close to violating, the cosmological principle, which expects effective homogeneity at these scales. This has partially contributed to arguments that structures like large quasar groups should not exist - and, indeed, that observations of them may simply reflect random clustering or so-called pseudostructures.

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Lisa Randall in her book Warped Passages points out:

I find the term multiverse a bit strange since the universe is defined as the whole that is the unity of its parts.

And this from someone who uses the term multiverse for brane world cosmology! The largest structure in the universe, is the universe itself. It's tautologically true...

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    $\begingroup$ The first rule of tautology club is the first rule of tautology club! I think the question is clearly about interesting sub-structures in the visible universe. For example, if someone asked about the largest structures in the Milky Way Galaxy, they would probably expect some discussion the central bulge and the spiral arms and the dark-matter halo, not to be told "the largest structure in the Galaxy is the Galaxy." $\endgroup$ – rob Dec 28 '18 at 3:36
  • $\begingroup$ @rob: Sure, nevertheless it is worth pointing out. There are, for example, some objections about the use of QM to model the cosmos as a whole because there is no outside observer. And if you take the notion of multi etale seriously - which I don't - then the universe per se is a large substructure. $\endgroup$ – Mozibur Ullah Dec 28 '18 at 13:50

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