So why would anyone think there is no such thing as Dark Matter?
If you are referring to Michael Mozina then that is not his position.
His position is that dark matter exists but that without "empirical evidence" it can never be SUSY particles that emit gamma rays that could explain features in the Fermi data.
He has his own personal definition of empirical evidence which seems to be only that evidence that can be tested in controlled experiments in labs here on Earth.
The actual definition of
empirical used in science and by scientists includes observations, i.e. most of astronomy, evolution and lots of geology. The definition also does not say anything about experiments being controllable. An observation of a supernova is empirical evidence despite the fact that we have no control over the supernova.
The empirical evidence is that dark matter is made of SUSY (or more exotic) particles.
Much of the thread is now about Michael Mozina's idea that dark matter must be normal matter, e.g. rocks. This is easily shown to be wrong. Planet sized (or bigger) rocks are ruled out by the search for
MACHOs in the Milky Way halo only finding at most 8% of the mass needed for dark matter.
Smaller rocks are ruled out by the frequency of collisions between them This turns them into either bigger rocks (if they stick) and so ruled out above or more likely into even smaller rocks with a higher collision rate. It also heats them up. The end result is an easily detected plasma of the elements of rocks (carbon, oxygen, nitrogen, hydrogen and others).
Another problem is the origin of the rocks. If 70% or even 96% of the universe (since Michael Mozina does not believe that dark energy exists) is rocks then where did they come from?
The elements in the rocks could come from supernovae but I doubt that the observed rate of supernovae could supply that much within the lifetime of the universe (13.7 billion years).
If by some magic supernovae supply the elements then you need stars to form to condense the elements into rocks. But only 0.4% of the universe is stars!
Once you have rocks as dark matter then you have real problems with the formation and evolution of stars.
- They will form from a mixture of rocks and the interstellar medium. Thus all stars will have a high percentage of C, N, O, etc in them (a high metalicity). This is not observed.
- It is even possible that the metalicity of the cloud of gas and rocks is so high that stars cannot form. What you may get is a hot plasma that is not undergoing fusion. After some millions of years you have a realy big and cold rock.
- IMO stars with a high metalicity will screw up the Hertzsprung–Russell diagram by truncating the main sequence at a lower color, e.g. no stars like the Sun.
And yet another problem with rocks as dark matter:
The large scale structure of the universe is well matched by the Lambda-CDM model (CDM = cold dark matter) using the current amount of dark matter. This structure is established early in the history of the universe, e.g. galaxies are formed before 1 billion years after the Big Bang. Thus all of these rocks must have also formed early.
So there is only a couple of billion years for some process to convert 70% or 96% of the mass of the universe into rocks.
This process then stops! Otherwise 100% of the universe could be rocks.
