Most of the Universe is missing and decades of searching have so far elicited no sign of it. For some scientists this is an embarrassment. For others it is a clue that might eventually push physics towards the next frontier of understanding. Either way, it is an odd situation.
Science has hunted in vain for the missing material. Its existence has never been detected directly, only inferred from hints. Yet if the rest of what we know about the way the cosmos is structured is right, it must be about five times more abundant than all the matter we can see in the Universe.