The Smith Cloud And Why It Matters
You may already have seen the press releases about the Smith Cloud, a galaxy-mass cloud of hydrogen (but no stars) interacting with our own galaxy. Researchers have found that to survive a collision with our own galaxy - which it seems to have done - it must have its own dark matter to bind it together.
What the press release doesn't
do, in my opinion, is explain why this is important. "Dark galaxies" have been a hugely controversial topic for years, so, if confirmed, this could be very significant. There aren't as many dwarf galaxies orbiting the Milky Way as theories predict (about a factor 10 less, in fact), and the idea that some of them don't form any stars is one way of explaining this.
Looking for dark galaxies was the original title of of PhD, so I explain more about it below. The image shows the Smith Cloud from the NRAO press release, made with the 100m Green Bank Telescope.http://astrorhysy.blogspot.cz/2014/05/the-importance-of-being-idle.html