Space: a vacuous void or an entity? I am a chemist and, until recently, a senior Research Manager with a major chemical company. Please excuse the potentially naive ramblings of an idiot in what follows, but I do not have the mathematical skills to formulate it otherwise; nor the inclination to spend several years developing them!
For some time now I have been struggling with the idea of an instantaneous ‘bang’ of an infinitesimal point, and why that might produce something where >>90% just turns out to be a vacuous void.
Just supposing ‘space’ is not a vacuous void.  Supposing ‘space’ itself is an entity of as yet, undefined properties; but which includes mass and an intrinsic contained energy.  To what extent would such a ‘space entity’ assist with the missing >90% ‘dark’ mass and the dark energy problems?  What would the mean mass and energy of space per unit volume need to be to account for the anomalies?
Might the distortion of the ‘space entity’ by displacement, caused by a massive object, then explain the space-time distortion and gravity; by an increase in some property (‘density’?) of the ‘space entity’ around the massive object, decreasing with the inverse square of the distance?
How might the presence of such a ‘space entity’ then effect the quantum mechanical world of particle physics?
 A: First the obligatory comment that the Big Bang didn't happen at a point. Put behind you all those representations of the Big Bang as an explosion, which are so beloved of TV documentaries.
Now on to your question: far more than 90% of the volume of (the known) universe is vacuous. The average density of matter is around 1 hydrogen atom per cubic metre while the density of a star isn't that different from water. So stars are about $10^{28}$ times denser than the universe as a whole. You'd have to take into account the gas clouds in galaxies, but even so the fraction of the universe that is effectively empty is going to be 99.a_lot_of_nines%.
But this isn't a fundamental property of the universe and doesn't need to be explained by some hypothetical property of space. Immediately after the Big Bang the universe was almost completely homogenous with no voids. The earliest evidence we have is the cosmic microwave background radiation, and this shows that 380,000 years after the Big Bang the density differences in the universe were less than 1 part in 100,000.
The empty space that we see today has developed in the 13.8 billion years since the Big Bang simply because matter attracts other matter due to gravity. So left to itself matter forms clumps with regions of vacuum in between them.
A: Your entity is pretty close to the conception of the modern conception of the quantum field in its ground (lowest energy) state. Space and time are not voids of "nothing", but are made of the quantum fields. What conscious beings mean when they say "things happen" in the World is that they are observing interactions between the handful of quantum fields that are our Universe. When we say there is an electron in a certain part of spacetime, we really mean that the electron field is in a certain, raised energy state. An atom here means the same kind of thing, only more complciated. It is our experimental observation that the fields change in steps and these steps are what give the "graininess" or particulate flavor to the quantum fields. In this conception, it is not always meaningful to talk about where particles are: in modern quantum field theory the "particles" are more like "notches" on tablets in some bank account bookkeeping the state of the quantum fields.

"Supposing ‘space’ itself is an entity of as yet, undefined properties"

Actually they are not altogether undefined. "Empty space" fulfils the vacuum Einstein equations, which means that different parts of "empty spacetime" have materially different properties: curvature for instance. I don't recommend the "rubber sheet" analogy commonly given, but instead try your hand at the "Holonomy" Wikipedia article: curvature defines how a vector transported on a closed polygon whose sides are geodesics change on each circuit, so in principle defines a real, physical, repeatable measurement we could make on different regions of spacetime and get repeatably different results.
Even so, Einstein's theory is what is called a classical theory, meaning it doesn't yet describe its results in terms of a quantum field. Eintein's theory is really a description of the "stuff" of empty spacetime without giving us any clue how or why spacetime has these properties. It could be said that modern quantum gravity research is the endeavor of learning to ken the stuff that "empty" spacetime is made of.
As for "dark matter", no, we don't believe that the "entity" of "empty" spacetime accounts for this. However, there is a cosmological constant in Einstein's equations which we experimentally find to have a nonzero value. This drives an acceleration of expansion of the Universe. Sometimes the cosmological constant is called "Dark Energy". I'm not a cosmologist, but my understanding of why it is not the dark matter explanation is that, although it can be construed to play a somewhat "matter" or "stuff"-like role in the Einstein field equations, the analogy is not complete because the "Dark Energy" has fundamentally different mathematical properties from the terms in the Einstein field equation that arise from "stuff", i.e. the quantum fields in their non ground states (I'm talking about the fact that its so-called covariant derivative vanishes, whereas that of the "stuff" terms - the stress-energy tensor- does not).
