What is the largest nebula?

As of 2014, the largest nebula ever discovered surrounds the active quasar UM287. It stretches across at least 2 million light-years and is illuminated from within by the quasar at its core. The discovery of this nebula might shed light on the structure of the early universe.

Standard models of galaxy formation predict the growth of immense filaments of gas and dust between galaxies. Usually, this gas is difficult or impossible to see, as it does not produce light of its own. In the case of UM287, however, the intense light of its internal quasar renders its extensive structure visible.