We're in hairy country here, and it's very easy to speculate using terminology that has a specific physics meaning, without understanding the underlying implications. We need to tread carefully. That said, some thoughts.
Until String Theory (or Brane theory) comes of age, it's nearly impossible to try to connect Quantum Mechanics (the wonderfully successful theory of the physics of the subatomic realm) and General Relativity (the wonderfully successful theory of the Universe on the large scale) in the same thought train, but both have useful insights. Right now, they don't easily connect, and it's a very active area of research.
There have been numerous experiments to measure the attractive force of gravity. It has most certainly been shown that one mass attracts another through gravity. That's how Newton's Gravitational Constant has been measured. (General Relativity simplifies nicely to Newton's theory of gravity on the human scale.)
So after the Big Bang (see below) the sudden immense expansion of the Universe gradally slowed due to the "drag" of the gravitational attraction of all masses in the Universe on all other masses.
Brian Schmidt (Mt Stromlo) and his colleagues have pretty good observational evidence that some billions of years ago, the gradual slowing of the expansion of the Universe after the Bing Bang started to go into reverse.
Incidentally, the Big Bang theory is almost universally (sorry, pun) accepted by cosmologists as the right description of what has been happening over the past 12 or so billion years.
Anyway the reverse discoverd by Schmidt and others has been attributed to the mysterious quantity called "Dark Energy". This fits easily into the equations for General Relativity, under the (rather too convenient) parameter Lambda in the equations for General Relativity. This was originally seen by Einstein as a fudge factor to keep the Universe stationary (which of course was a mistake).
What Dark Energy is remains obscure. Vacuum Energy (as in the Casimir Effect) seems to be related.
On the microscopic scale, Quantum Mechanics provides some interesting insights, too. In the Standard Model (widely accepted and very relaible as a predictor of the behaviour of matter and energy at the quantum level), the several forces evident in nature are associated with "elementary particles". The particles are said the "mediate" the forces. For example, the particle associated with the Electromagnetic Force is the photon.
Not all the associated paticles have been found. The force of Gravity has an associated (hypothetical) elementary particle called the Graviton. This particle has not yet been created in any of the Particle Accelerators (CERN etc).
One of the more elegant ideas about the expansion Schmidt and others have found from their supernova work is that the Graviton might have some mass. It is conventionally assumed that the Graviton has no mass, like the photon.
However, if it does have some mass, this implies (through complex maths) that the force of gravity starts to "let go" beyond a certain distance, and this fits - albeit qualitatively - quite nicely with the observed "Dark Energy" expansion.
We have a very long way to go before understanding what is going on out there (or in there), and it's a very exciting time to be a High Energy Physicist or a Cosmologist. And very confusing for the rest of us
Good sources of further information are the regular articles on these subjects in New Scientist magazine. Wikipedia is another excellent place to look. See for example the info on the Graviton at
http://en.wikipedia.org/wiki/Graviton
DN