Dark matter

Because there is more of it near galaxies. Or rather, the galaxies themselves form where the dark matter clumps together under its own gravity, because the gravity of the dark matter causes normal matter to "pool" there as well.

Because it isn't very dense. Sure, most of the mass of a galaxy is probably in its dark matter halo, but it doesn't appear to clump into dense objects like stars and planets. When you spread out all that mass across the vast amount of empty space available in and around a galaxy the density becomes much too low for one to notice any local gravitational effects that might exist.

 

Jay, consider a very large cloud of Hydrogen gas in the galaxy. The combined mass of this gas is easily 1000 times more than our Sun. As you are passing through this cloud you would barely be able to detect the gravity because the density of the cloud is very small and it is all around you. Only by measuring things such as a star passing by the cloud would you notice the gravity because it is being pulled towards you. Similarly dark matter is spread out all around the galaxy instead of being clumped together, so we have a very difficult time detecting it within our own galaxy. When we look at other galaxies we can see the large scale effect because we are outside of that galaxies dark matter and can see things such as gravitational lensing and the rotation of the stars in the galaxy.

 

Has dark matter been tentatively discovered?, is it 3 phase? Can someone confirm what I read about the ammonia tank detectors a while back?