Just a small point ( I really am quite literally running out the door, er through the door, er doorway)...have to carefully what you say these days
I say...that the origin is far less important than the future, glib as that may sound, what it really means is that to find the answer from the past, one must look toward the future.
How will it end?
Will it end?
What do you mean by end?
I have for 8 years pondered the same questions that have been discussed herein and I have thought that this entire phase (Origin-Expansion-End) is just another lap on the treadmill of existence.
I think conventional thinking is too limited, I fear we are blinkered in how bold we dare to think.
If existence were continually born, lived (existed) died, then would not this be analogous to say the blood being pumped into the human heart. Consider that blood can only flow one way (in a working heart, not a diseased one), and a unique filter exists which permits the flow of blood in only one direction.
The origin of our universe, or if you like, this existence, this time around, began at the point where the filter operates, some call it the big bang, others a quantum singularity, the name doesn't really matter.
What matters is that the eternal process has begun again, simply another lap, in the unending cycle of existence.
The passage from one side of the filter to the other (from a previous existence to this existence) can be a monumental event in our minds, but forgetting the energy and the forces at that moment, it is just the way it is.
All birth comes with some pain (ask any mother), so when a new existence begins, there is a little tumultuous grunting and groaning (so to speak), please permit my flowery and colourful language be used here, it is only meant to illustrate a point.
At the end of each cycle (what to us would be an inconceivably tumultuous and cataclysmic event) is little more than the passing through the one way linear filter to begin the next cycle.
Thus I humbly submit that there is no begin or end, just being, just existing, and the markers we use, are there to give us some sense of where we are in the current lap (cycle).
To think that something came from nothing is IMHO unlikely, thus to eliminate this from my mind which cannot cope with the constraints placed on it, I have totally omitted the idea of beginning and end and converted it into continuous and unending. Who can say what lied before the current lap began and who can say what will transpire after this lap ends and the next one begins?
This is perhaps over simplistic, but nature (existence) is IMHO simple at it's core, it's 'us' that looks to explain things in a manner we can easily understand, and I agree that is a noble task, but perhaps opening ones mind and truly looking at things that don't stem from traditional teachings might be as powerful as tertiary learning.
I wish I could write more, but I have to run. I did quite a few essays on this when I went through Swinburne years ago, I only regret not keeping it, I had some pretty radical ideas back then, just the type of ideas that actually lead to new understanding

I'm glad I'm all grown up now
Have fun with the thread boys, of course you are all correct