[g__day (Matthew)]

I'm pretty sure gravitationally bound structures aren't expanding - so atoms aren't getting bigger - nor the distance between them, its the spacetime fabric or framework between galaxies - which does not even have to be relativistically bound - which is said to be expanding.

Put simpler - where gravity is extremely weak - the framework of space and time that gravitational bound structures like galaxies and cluster hangs in is in itself getting more diffuse and larger. We don't know for a fact what physical frameworks hold dominance in the spacetime between galaxies - not even relativity is proven to be the dominant force across the vast emptiness of spacetime.

So when physicsts say 95% of matter is missing - the provisors should be given:

1) assuming relativity holds as the dominant framework for non gravitationally bound (empty) spacetime between galaxies
2) given what we observe is typical of the entire Universe (assumes homogenity in every direction - or that our corner of reality isn't an odd one)
3) it is not an exotic after effect of distorted geometry for spacetime itself arising from the aftershocks of the big bang and inflation

They are three very big, very untested provisors that we have no way of knowing hold true. Change a single one and our understanding of reality and missing matter changes totally. Einstein only postulated the second so the maths would be workable - with out that hugely simplifying constraint the field equations couldn't have been solved with only 20th century maths. Our mathematically frameworks have only been powerful enough to model two black holes interacting and merging in the last two years! Go back to the 1920s and 30s and there was no chance to solve the general field equations without alot of simplfying limits that were simply guessed at as reasonable.

Food for thought at the very least!

PS

The edges of our Hubble sphere (light cone) for the Universe, given a calculated age of 13.8 +/- 0.1 billion years - varies from 40 billion to 200 billion light years depending on what variant of the laws of physics you model as interacting to shape the geometry of expanding / unfolding (originally inflating) space time itself. Change how matter and energy curve spacetime, and the fall of rate (e.g. MOND) and you get wildy different geometry. Make spacetime 10 or 11 dimensional or allow super symmetric particles (s-particles as a form for instance of cold dark matter e.g. super massive, uncharged, slow s-neutrons) and you get different geometries again.

The key to what we have to unlock is the geometry of spacetime and what framework of laws shape it. To know that we need to study high energy events and postulate every more complex laws of physics. The best two I have seen lately is 1) scale relativity and 2) a variant of loop quantum gravity which has a tight constraint on matter and time - that being that time can only flow foward. Adding that second rule alone allows for the creation of large scale structures (galaxies) we see today in super computer simulations in a majority of cases. Without that rule (that no branch of physics currently enforces as a boundary condition or law) simulations of creation end in a jumbled mess of dimensions without structure - you can't it seems even force it to remotely create a universe like which we see today (Scienctifc America - July 2008 pg 24 - 29, the principles of casuality - The Self-Organising Quantum Universe- Jan Ambjorn, Jerzy Jurkiewicz and Renate Loll).

So that leaves theoretical physicists asking what governing principle in the framework exists - that we don't know about yet - is it that forces time to be one dimensional and directional (forward). Once we know what causes causility to be an organising principle of our Universe at a Qauntum level - we may understand its geomorty and topology alot better!