Dark Energy and Dark Matter
The universe is a violent place and energy abounds in it. Huge amounts of matter are continuously expelled from stars and other objects. Massive solar flares add to the great volumes of matter hurling out into space in the form of solar winds. Super Novas, gamma ray bursts, pulsars, and black holes generate enormous amounts of matter and energy and that energy radiates out from each and every galaxy in the universe; this is the intergalactic wind. In the vast regions between galaxies the intergalactic wind flows unimpeded toward the next galaxy with constant and great amount of force.
Consequently when the intergalactic wind encounters another galaxy it exerts pressure on that galaxy. From all parts of the universe each galaxy is receiving pressure from the intergalactic winds pummeling it. Were the universe somehow bounded or if there were an endless array of galaxies everywhere in an unbounded universe, the pressure of the intergalactic wind would have little or no impact on any galaxy. It would all balance out.
However the universe is not bounded and out there somewhere, in a place we are poised on finding, there are no more galaxies. At some point we will find galaxies that beyond which there is simply nothing.
These outermost galaxies at the edge of the universe receive the constant pressure of the intergalactic wind exerted only on one side. There is no pressure being exerted on the opposite side of these galaxies. Over time then, gradually each of these outermost galaxies will slowly be moved by the wind out into the vast nothingness that borders them.
In this manner, the extent of the entire universe expands. Further, toward the center of the universe, there is a lighter pressure of the intergalactic wind coming from the edge of the universe with its lesser concentration of galaxies. Thus all of the interior galaxies of the universe will slowly migrate outward in the direction of the lesser pressure. Hence the space between all galaxies expands.
As each galaxy moves a greater distance from its neighbors there is a lessening in the gravitational attraction between galaxies. Because of this reduction in the gravitational attraction felt by adjacent galaxies and because no new matter is being created in the universe, the expansion between galaxies will accelerate as the physical size of the populated universe expands and occupies more and more of the emptiness around it.
However, stars within a galaxy as well as local groups of galaxies that are bound by their mutual gravitation will not expand. They will continue to remain much the same size as we find them today.
