Hello,
Astronomers have used the gravitational tug of a huge mass on a more distant path of light to detect a large celestial object. What makes this significant is that this technique allows astronomers to locate objects i.e. galaxies or galactic clusters in their proper place without having to study thier light. The article makes a good point : most of the "big action" in the observable universe is governed by mass of objects not their visible light or other electromagnetic radiation and most of the matter in the observable universe is dark.
Read more :
www.space.com/scienceastronomy/astronomy/galactic_cluster_010815.html
see ya, VINCE