Personally, I like the idea of the Helix Universe. What this describes is that the size of the Universe, which at this moment in time is expanding, will eventually reach a maximum, and contract again (it is feasable that time will begin to flow backwards here too). When the Universe reaches a single point of matter again, it begins to expand once more, in a never ending loop. A diagram of this looks roughly like a Double Helix. In this way, energy is conserved, yet the Big Bang can still exist.
Remember, outside the Universe, time does not exist. The Universe contains the dimensions that we experience, and so "outside" it, there can be no time. Therefore, we cannot really talk about a "start" of existance anyway, since before the Universe, time cannot exist.
This is why I like this theory, it makes so much sense of all of these things.