Baryons are the sub-atomic particles primarily responsible for an object's mass. They are named after the Greek word βαρύς (barýs) for 'heavy'. They include protons and neutrons. When cosmologists calculate the amount of baryonic matter which the universe should contain as a consequence of the Big Bang (using the maths of the Standard Model), the result does not correlate with actual cosmological observations.

Roughly half of the baryonic matter which the calculations predict appears to be 'missing'. The so-called 'Missing Baryons Problem'.

Various conclusion have been drawn, all with profound implications for cosmology and particle physics. Either :

1) The maths of the Standard Model is wrong

2) The observations are wrong

3) The missing matter has been altered in some unknown way by some as yet unknown process.

Some research groups are now suggesting that some of the missing baryonic matter is, in fact, observable. See a 2017 paper published in arXiv Missing baryons in the cosmic web revealed by the Sunyaev-Zel'dovich effect

Note: The 'Missing Baryons Problem' is separate and distinct from the 'missing matter' as implied by Dark Matterplugin-autotooltip__plain plugin-autotooltip_bigDark Matter

"The nature of the dominant component of galaxies and clusters remains unknown."

Source : Measuring the dark matter equation of state (Mon. Not. R. Astron. Soc. 415, L74–L77)"

In the 1930s, astronomical observations of galaxy rotations showed that the outer regions were rotating (about the galaxy's 'centre') at the same speed, or faster, than the central regions. Subsequent calculations referring to the galaxy's mass, and thus its internal gravitational attractions, showed that i… theories.

Also see : Proton Mass Calculationsplugin-autotooltip__plain plugin-autotooltip_bigProton Mass Calculations

The mass of a proton has only been calculated to an accuracy of around 4% - (roughly 938 MeV/c2 or 1.672 × 10-27 kg). The constituent particles - quarks and gluons - which make up the proton, have individual masses that add up to only around 1% or so of its measured mass (which can be determined accurately with specialised devices called