Where is the universe hiding its missing mass?

Black makes a person look thin, people like to say. The universe apparently knows this trick too. In any case, it looks significantly skinnier to astronomers than they think it is in reality. Scientists think this because they can calculate how much matter was originally released by the big bang (this refers only to normal matter, not dark matter). If we add up everything that we can see in the sky today, however, we only get to two-thirds of this original amount.

Where is the universe hiding...