= Gas Filaments Between Galaxies =
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Incomplete!  Notes will expand as I puzzle my way through the papers.

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Both of these papers overlay Planck images of about a million galaxy pairs in order to produce a "signal" with 5 sigma statistical significance.  

I wonder how many other phenomena lurk out there, too weak to observe directly with current space probe capabilities.  My guess is some will require space telescopes with enormous mirrors, very far from the Sun.  I suppose there will always be a need for vastly better telescopes, and jobs for astronomers into the very distant future.  

Never attribute to exotic invisibility what can be adequately explained by observational insufficiency.  We should attempt to quantify the huge bounds of "what we can't see", and guestimate the magnitude of the monsters that will remain unobserved until our instruments improve.

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 * https://arxiv.org/abs/1709.10378
 * Missing baryons in the cosmic web revealed by the Sunyaev-Zel'dovich effect
 * Anna de Graaff, Yan-Chuan Cai, Catherine Heymans, John A. Peacock

Can account for 30% of the total baryon content of the universe, about 3 times the visible baryon content.

The Sunyaev-Zel'dovich effect is the Compton scattering of the Cosmic Microwave Background (CMB), 
so its effects can be inferred from Planck space telescope data.


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 * https://arxiv.org/abs/1709.05024
 * A Search for Warm/Hot Gas Filaments Between Pairs of SDSS Luminous Red Galaxies
 * Hideki Tanimura, Gary Hinshaw, Ian G. !McCarthy, Ludovic Van Waerbeke, Yin-Zhe Ma, Alexander Mead, Alireza Hojjati, Tilman Tröster

Hubble dimensionless constant h is around 0.7, which means the Hubble constant Ho is around 70 (km/s)/Mpc .  A fudge factor to compensate for differing measurements and opinions.