This was the general contour of the debate for a number of years. It was assumed that immediately after the big bang and as a result of the big bang, space and matter was expanding away from each other at fairly rapid velocity. However, the gravity due to all the mass in the universe was pulling matter closer together, thus slowing the rate of expansion.
The mathematical question that physicist were trying to resolve was the density of matter in the visible universe great enough (above a certain threshold) to slow and eventually stop the expansion of the universe so that the universe eventually came back and contracted in on itself (the Big Bounce) or was the density below that threshold so that the universe would continue expanding forever (the Big Freeze). this is analgous to whether a space shuttle is moving fast enough to escape Earth’s orbit or would eventually be brought back down to earth.
For many years, this was the assumed model of the universe, and physicists were studying the motion of galaxies and the night sky to try to figure out which of these endings would occur.
However, quite recently (within the last 10-20 years or so), physicists made a shocking discovery that the rate of expansion of the universe was not actually slowing, but it was
increasing. What this means is that there must be some other as yet unknown force in the universe serving to push the universe apart at every faster speeds. The term for this force is “dark energy” (as Jamma briefly refers to). Really, dark energy is just a name or a placeholder for that phenomenon that we observe, and we don’t have any idea what it is or what causes it (although there are a number of hypotheses).
Sorry for the tangent. But, God’s creation is so awesome and always more amazing than we realize. I had to add on to Jamma’s excellent explanation.
