Dean's World

There's a lot of head scratchers out there. Another one is the spiral arms (streaming clusters of stars at the outer edges of ours and other galaxies) are moving to fast to keep from being thrown off from the their galaxies.

There just ins't enough mass (gravity) in the galaxies to counteract their centrifical force. So they make up the difference for the unknown X and give it the name dark matter, and/or dark energy.

That is not to say that I don't think dark matter exists, just that no one really knows what is going on.

Sandi's a bit confused about the role of dark matter in galaxy dynamics. It's not true to say that spiral arms are rotating too fast to be gravitationally bound. What is actually happening is that if you look at what physics (ordinary Newtonian physics at these speeds) tell us, the angular velocity of stars in a galaxy should drop off as you proceed away from the centre. The problem is that observations do not show this. The curve does not fall away in the way that physics predicts. So astronomers adduce a halo of matter that is not detectable by electromagnetic radiation.

I think dark matter and dark energy almost certainly do exist. A gravitating vacuum is not ruled out by any current theory. Special relativity is among the most rigorously tested and confirmed theories we have. General relativity is very robust. We do know that it breaks down at very high energies (i.e. very small length scales), but for most cosmological phenomena it's amazingly well-confirmed. If it is eventually overturned, it will be in the way that relativity overturned Newtonian mechanics: by incorporating it as a boundary case of the wider theory.