Re: Theoretically, would enough neutrinos together be visible?

The real answer to your question is that probably nobody really knows enough 
about neutrinos to give a sensible answer...  However, that's never stopped a 
physicist from venturing an opinion, so here goes...

First, remember that neutrinos are generally viewed as massless particles of 
zero physical extent that travel at the speed of light and interact with other 
forms of matter only through the weak interaction -- not electromagnetic forces, 
nor gravity, nor the strong nuclear force.

If you believe all this, then you pretty much have to believe that you couldn't 
"see" even a giant swarm of neutrinos if they got between you and the sun, 
because there's no interaction known between them and light.  On the other hand, 
if the giant swarm of neutrinos started interacting with some other stuff -- 
nuclear stuff, I think -- it might be possible that the second-hand effect of 
the neutrino/nuclear stuff interactiion might be observable in the visible part 
of the spectrum if some minor (unknown to me) perturbation of the electronic 
configuration of the atom occurred.  Maybe...  But the chances of collecting 
enough neutrinos (outside of in a black hole where God-knows-what goes on) to do 
much more than allow scientists to do enough research to publish papers and 
understand a little more about how the Universe works on a small scale is really 
pretty small.

Not to demean your question -- because both my wife and I really liked it 
because it made us think about things we'd never conjured before -- I really 
think that all of this stuff falls into the category of "it doesn't matter 
because it couldn't possibly happen,"... sort of like the question of what would 
happen if enough people jammed onto the San Andreas Fault in California and 
jumped up and down, would it cause Los Angeles to fall into the Pacific:  it's 
not clear how much is "enough" and how you'd ever get "enough" Californians to 
agree on something to do anything together on such a large scale.  [I can say 
that, because we lived in California for 20 years.]

Thanks for the question, and keep 'em coming... but don't worrying about the sun 
going down behind a cloud of neutrinos!