A couple of years ago, it was found that there were some streamers of
gas that were flowing between Andromeda and Triangulum, which scientists
thought were the result of a possible near collision between them.

Neighbor galaxies may have brushed closely, astronomers find
http://www.sciencedaily.com/releases...0611193632.htm

But now, after some further analysis, they are finding that there may be
something more significant here. They think the gas between the galaxies
are as massive as dwarf galaxies, but just more diffuse, so there are no
stars that formed in them. So the nearly invisible gas is likely to be
independent from either galaxy. What's more, they think that this gas
came from an even bigger clutch of gas that has so far remained invisible.

Astronomers discover surprising clutch of hydrogen clouds lurking among
our galactic neighbors
http://www.sciencedaily.com/releases...0508131700.htm

If this is true, then it's making it more and more unnecessary to look
for exotic WIMPs and other such things to explain dark matter, as dark
matter may simply be this invisible gas between galaxies.

Yousuf Khan