"P" == Painius writes:

P "Joseph Lazio" wrote... in message
P ...

P Perhaps we can train the powerful Hubble on the central area of our
P Local Cluster? Maybe there are distant galaxies that will be
P distorted by the gravitational lensing of dark matter at our own
P galaxy cluster's center?

The problem is that we are not in a "real" cluster. We are in a
small group of galaxies, composed of the Milky Way, the Andromeda
Galaxy, M33, and various dwarfs. The total mass of this group is
perhaps 100 times smaller than the typical large cluster from which
gravitational lensing is seen. Moreover, we are sitting in the
middle of it.

P Thanks, Joseph... not sure how you come by that we're in the middle
P of the LG. The following, while depicting us in the center,
P actually shows that the grouping is quite off-center. For the most
P part, the center appears to be off to the right and down from our
P galaxy...
P http://www.anzwers.org/free/universe/localgr.html

Depends upon how you define "middle." The Milky Way is one of the
three dominant galaxies in the Local Group. Moreover, the Milky Way
obscures our view of a good chunk of the sky, and some of these dwarf
galaxies are quite faint and can be difficult to find. In all
likelihood, the Local Group doesn't have a well-defined "center," but
three, nearly equal, large gravitational wells.

The nearest cluster of any reasonable heft is the Virgo Cluster.
I'm not sure if anybody's ever looked for gravitational lensing
from it, but ADS or astro-ph will likely contain a reference if so.

P Could not find anything in ADS, but astro-ph is working on it...

P http://xxx.soton.ac.uk/abs/astro-ph/0003422

P So Joseph, the nature of centro-cluster dark matter leans me toward
P lots of rogue black holes circulating along with the galaxies.
P Probably lots of activity and collisions. We may be very fortunate
P to be in the outskirts!

I'm somewhat more optimistic than you. Remember, space is big. Even
if there are lots of black holes (and there are some good reasons to
think that not all dark matter is in the form of black holes), there
won't be that many collisions.

Also, to provide some perspective on the limit quoted in this paper
(optical depth ~ 10^{-3}), the microlensing optical depth toward the
center of the Milky Way is only of order 10^{-6}.

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