In article ,
Hans Aberg wrote:

What would happen if the matter is very thin infancy matter, of the kind
known to form very young stars? Would that be easily detectable, or
possible to rule out?

I don't know the term "infancy matter," but it sounds like it means
stuff that includes a lot of hydrogen gas. If you try to put
a large amount of diffuse gas in the Sun's neighborhood of our Galaxy,
I think it'd be pretty easy to spot. In particular, it'd produce whopping
great absorption lines in the light from nearby stars.

I think you're right that it'd be blown out of the inner solar system
by the solar wind, by the way. That's why I think that the main way
to look for it would be on slightly larger scales.

So if you want to put enough hydrogen- and helium-rich stuff in our
neighborhood to make a significant contribution to the dark matter,
you can't make it diffuse. You might try to stick it in
gravitationally-bound (Jupiter-ish) lumps. That gets around the
absorption line problem. You can detect such lumps in other ways,
especially by the technique known as gravitational microlensing. The
limits set by this technique show that some such objects exist, but
that there can't be enough of these lumps to make up all of the dark
matter.

-Ted


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