I think that's why it's called space..... it's mostly empty

My understanding is the current theory has to do with the filaments of matter around and between galaxies. It's basically matter we can't "see" directly because it's so diffuse, so appears to be non-baryonic, but is actually baryonic. I can't recall the exact stats and am at work right now, but this allegedly is actually able to account for a huge percentage of the remaining mass. Maybe someone can back me up or refute this.

I'm not really sure what you mean by "Negative mass hypothesis". The 95% of the universe thing is true but perhaps a bit misleading. We don't really understand "dark energy" so we can't really say what it is or how it may act in the future. What we do know is that something appears to be needed to explain the structure of spacetime that we see in our universe. If we assume that GR (General Relativity) is basically correct on the large scale, then dark energy is the most significant factor in explaining the large scale structure of spacetime.

I personally favor the cosmological constant explanation which extends Einstein's GR. So if GR is right and there is a constant in the equations that underpins the acceleration of the expansion that we observe then some other stuff fits in an elegant way.

I suspect that we will soon solve the mystery of dark matter. My guess at the moment is that dark matter is one of the long sought super-symmetric partner particles. Within the physics community there are hints that something along these lines is on the threshold of discovery.

I am indeed really interested. I hope that the answer isn't some of the simpler ones that I have heard like "tired light" or something else that is sort of mundane - at least from the perspective of trying to understand the grand design of our universe. My colleague at work (also a physicist by education, as am I) suggested recently that a tired light hypothesis had recently gotten significant support and that the accelerated expansion might go away. I hope not! I want to see exotic physics which may hint at a deeper theory that would significantly further our understanding and our search for a TOE (Theory of Everything.)

Good stuff!

The idea itself has merit -- and it makes "impossible" things like a perpetual motion machine or even a Warp Drive easier to construct... At the same time, it presents problems like runaway motion that need addressed before the idea is compatible with "normal" Newtonian physics.