[RossA]

I don't think this is what these scientists have discovered but gravity is definitely detected remotely. Massive galaxies (probably with black holes) bend light passing near them so we see multiple images of the galaxies behind them. Many examples have been posted on the "astronomy picture of the day" site - a worthwhile site for 2 minutes per day of learning.

[LibKill]

Very good. And I know that light bends around a massive object (please don't expect me to explain why, I could never do the math).

I called upon Physicist because he has a real ability to explain these things to a poor barbaric SOB like me.

[LibKill]

P.S. APOD is one of my favorite sites, every day.

[Physicist]

FWIW, I don't think that gravitational lensing plays an important role in this analysis. There's a lot of matter in these streams, but it's not as concentrated--by many orders of magnitude--as it is in a galaxy. What it lacks in density it makes up for in volume, however.