Concept of eternity.
Think of a sheet of paper with a string passing through the middle of it. The string seems to go on forever, off to the right (the future) and to the left (the past). It represents 1 dimension (left and right). The sheet of paper is two dimensions (up and down & back and forth). The sheet of paper like the line, goes on forever so that you cannot see the edges. This two dimensional sheet is traveling along the line, moving to the right (into the future) and the point at which the string intersects with the sheet is now (the present). The string represents our time and the sheet represents eternity.
We cannot speak of time, we must speak of space/time. Time and space began together.
If we did not live on the arm of a spiral galaxy but instead lived near the galaxtic center we would not be having this conversation. The intensity of the radiation does not allow for life.
Also the early universe was much hotter, and if the universe were not expanding the radiation would not be dissipated and the night sky would be 300 times brighter than the sun.
The lastest theory being tossed around is that the universe is really a multiverse (brane theory).
The big bang may have ocurred when two undulating membranes touched at a point. The universe has been expanding ever since. Either it has enough mass to eventually stop expanding and collapse again (a closed universe) or it will expand forever (an open universe) as it turns out the the universe appears to be balanced right between these and has been said to be flat.