[stephengillie]

Think of 4 dimensional spacetime as an oddly-shaped rectangle, which is much wider at one end than another. Maybe more like a pyramid with the top bit missing. All of matter and energy started as an explosion at the narrow end, and is traveling across towards the much broader end. The expansion of the universe is merely the expansion of the barrel, and the dark energy spreading apart the universe is really the slight trajectory differences caused by the shape of the charge that started the process.

To me, it's like 3d canon shot flying down a 4d barrel.

Or trace the worldlines of these particles. Any given point in time represents a 3d slice of the 4d pyramid-rectangle.