Phillip Helbig (undress to reply) wrote:
In article , Ray
Tomes writes:

Gas and dust is able to be detected by its effect on light. Large
objects glow from their own light. If intermediate objects were at
low temperature would they generally be detectable? There are many
orders of magnitude between dust and the dimmest starts. How many
of these could definitely not contribute to dark matter and why?

"Dark matter" is a bit of a misnomer. What most people mean these days
is "some sort of non-baryonic matter, other than neutrinos, which we
have not yet detected". This is the "DM" in, e.g., "LambdaCDM". Other
things can be dark, such as cold gas, dust, and so on. However, we have
good upper (and lower) limits on baryons from big-bang nucleosynthesis,
so while there are some dark baryons, they cannot be an appreciable
fraction of the dark matter.

It's also a bit of a misnomer since "dark" here means that it does not
interact electromagnetically. Yes, that means it doesn't glow, but it
also means that it doesn't reflect or absorb light. In other words, it
is transparent. At most it is weakly interacting (i.e. via the weak
"nuclear" force), but this is not something which can be investigated
astrophysically, so only gravitational effects are left. (Since we
don't know what it is, we don't know if it is self-interacting and hence
don't know if it can form macroscopic objects.)

What intrigues me about DM is that beyond some point from the galactic
center the sum of luminous and dark matter, as a function of r, is
constant. Just how far out does that hold? Could this constancy be
a result of galactic formation or could it be true everywhere?

Could DM be proto baryonic matter?