But the planet Neptune is not some 'exotic' form of matter, so it can't be considered Dark Matter.
To re-iterate, “Dark Matter” is an all-encompassing generic term used over the years covering cases where we see evidence of gravity before identifying the mass or masses responsible.
Don't confuse the terminology used to describe the modern Dark Matter problem with the underlying concept of matter being detected by indirect means, in this case, gravitationally, before it could be directly detected. I say Neptune was 'Dark Matter' in quotes to indicate it was not literally called Dark Matter by the researchers of the day. However it does still meet the requirements of Dark Matter, that it was detected through its gravitational influence before it was detected by more direct means.
Today we continue to find remote members of the solar system that were previously below the detection threshold of our instruments. We also are finally developing an inventory of extrasolar planets, some detected initially by their gravitational influence. These are also components of Dark Matter (the baryonic component).
So what qualifies as 'exotic' matter?
For a time, Dark Matter had a far broader range of definition, which included baryonic (Wikipedia) matter. It is only fairly recently, as more of the baryonic components are identified, that the definition has narrowed in on a subatomic particle.
Does the neutrino qualify as 'exotic' matter? The neutrino is non-baryonic, as are electrons. Neutrinos are suspected to be just one of the possible components of Dark Matter.
If one wishes to claim that 'dark matter' is nonsense, the statement carries with it the implication that our current level of science and technology is at its peak and there is nothing which our current technology cannot detect. That is:
- We know how to detect all types of subatomic particles in the universe, no matter how they interact with other particles that we know.
- Our telescopes can detect all matter in the universe by the light it emits. There is nothing below the level of sensitivity of our current telescopes.
The history of science and astronomy has shown that assuming nothing can be beyond our current technology's level of detection is a losing bet. Every time we've had dramatic increases in instrument sensitivity, we've made new discoveries of what is 'out there' and sometimes new discoveries on smaller scales of size as well.
As I noted in an earlier post, Theory Vs. Experiment. II, there is a certain symmetry in the possible existence of an additional class of particles if we group the particles by the interactions to which they respond:
|Electrons muons, tau|
Such a pattern, if real, might suggest a new avenue for the whole Grand Unified Theory (Wikipedia) option. After all, even I am beginning to think string theory is stretching to the point of breaking.
Dark Matter is a Hack
In some ways it is. But it has the advantage of being a very simple hack, just an additional particle that only interacts via gravity. Even better, it is a TESTABLE hack. In your simulations, you add an extra density component that only responds to the gravitational interaction and see how it changes your results. Though this process, Dark Matter has made a number of successful predictions detectable in astronomical observations (such as the Bullet cluster, Wikipedia).
Another advantage, compared to some other alternatives to Dark Matter, is that a previously undetected particle has the potential of being demonstrated in laboratories (XENON project).
Realistic Alternatives to Dark Matter
Numerous alternatives have been proposed to solve the missing mass problem. Some, such as Anthony Peratt's galaxy model, have already been ruled out by more recent observations by instruments such as COBE and WMAP. I've written much about this model on this site as it lives on among Electric Universe supporters.
Modified Newtonian Dynamics (MOND): At one time I regarded this option as borderline 'crank' science. In recent years, the supporters have actually been producing mathematical models that are actually *testable* against observations. Unfortunately, unlike the possible particle component of dark matter discussed above, it is unclear if MOND could ever be tested at laboratory scales. (Wikipedia)
Relativistic Effects due to Matter Inhomogeneities: I include this possibility since when I first read about them, I thought it was a cool idea. Basically, some aspects of cosmology rely heavily on the universe being very smooth or uniform density on large scales. But what happens if there are large non-uniformities? There were some interesting papers suggesting that the gravitational self-energy (the gravity created by effective mass density of gravitational energy) could distort space-time sufficiently to mimic the effects of Dark Matter. The last I heard, these have been dismissed as mathematical errors.
Next: Just how much 'dark matter' do we need?