Uncle Al's response to this is very interesting.
I know that everything that is said about dark matter is speculative, but there are a few questions that can be raised.
Dark matter (DM) is supposed to interact with ordinary matter only by means of gravity. With no radiative interactions with the visible matter (VM), why is DM not blazing hot? Unless, of course, it was formed after the Big Bang. This could be regarded as raising a question or giving a hint. (Hints about formation and properties.)
The other idea that Al is raising is that two balls of DM pass through one another with no obvious distortion. Is it acting like a point mass? No. Is it acting like a cloud of (dark) particles? No. Is it acting like a continuous fluid? No. Hmm, ... So does that means that DM doesn't interact with DM via its own gravity?
I'm starting to get the idea that DM is like the luminiferous aether of the 19th century. It seems to have many mysterious properties (rigidity, lack of motion, etc.) and it cannot be detected just like a subtle imponderable fluid. Could it be - gasp - that our understanding of gravity is not up to par?
The hypothesis of DM (and dark energy) can explain some things and that is well and good. However, can it, in principle, be falsified? One could ask for other alternative hypotheses. Are there any?
I suspect that when other facts become clearer that some of the dark stuff will go away. The facts I have in mind include AGNs and galactic structure, current mysteries about galactic evolution, certain results in particle physics and the like.
I am not trying to be negative - I am hoping for new physics!
Dr. R.
