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?

[[Mod. note -- It's almost impossible to say that something does
"definitely not contribute"; usually "all" we can do is put upper
limits on any possible contribution. Gravitational-lensing observations
are often used to place such limits, and we've discussed them numerous
times in this newsgroups.
-- jt]]