Our own Milky Way is huge at 100,000 light years across, yet is dwarfed by the so-called giant elliptical galaxy which can be up to ten times its size.
Giant ellipticals are the biggest galaxy type of them all. Curiously, giant ellipticals are always situated in the denser regions of space. They are framed by up to hundreds of other galaxies. Each one of these other galaxies has its own 10-100 billion stars replete also with star forming gas and a supermassive black hole.
As the giant elliptical is so massive and thus has stronger gravity, these other smaller galaxies fall in toward the giant elliptical.
Like some bad science fiction movie about a "blob," the giant elliptical has a chance to grow by cannibalizing its smaller galaxy neighbors. The giant elliptical tears apart each nearby galaxy, separating out the stars from the gas, the dense nucleus, and the supermassive black hole.
Interestingly, the supermassive black hole is thought to settle quickly in the center of the giant elliptical which just 'ate' it, thereby growing the size of the cannibal's supermassive black hole stomach. As evidence that this cannibalization process actually happens, in some cases the remnants of 'undigested' galaxies can be spotted inside of giant ellipticals against the vast panoply of its constituent stars.
Given that giant ellipticals are the sites of violent collisions with slow cooling times and are situated in hot environments, such objects are thought to be unlikely places to form new stars. Even so, recently astronomers have found considerable amounts of cold gas typical of the sites of new star forming regions surrounding some giant ellipticals.
The cause is still unknown, with some led to believe that the large and growing supermassive black hole may provide a kind of thermostat which regulates the heating and cooling of the galaxy that surrounds it.