On Oct 14, 8:27*am, wrote:
In sci.astro Uncle Al wrote:

By hypothesis and definition dark matter does not
interact except by gravitation. *

Not true. *By hypothesis, dark matter interacts weakly,
but there is certainly no requirement of no
nongravitational interaction.

For Dark Matter as WIMPs, this is true. But the Bullet Cluster
obviates such weakly interacting Dark Matter, doesn't it? The visible
matter is pretty hot, so maybe we cannot evaluate how much normal
matter is there...

*In fact, popular candidates (lightest supersymmetric
particle, axions) certainly do have nongravitational
interactions. *In fact, a major experimental effort is
going into searches for dark matter through such
interactions.

Isn't that a bit like searching for a lost item under a streetlight,
even if we did not lose the item there, because we can at least see?

David A. Smith