Dark matter

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.