Scientists have been playing a game of hide-and-seek for decades. They’re looking for most of the mass of the universe, which is hidden from view. So far, they haven’t found it. So they keep widening the search – coming up with new explanations for what the missing mass might be, and how they might find it.

Their quarry is dark matter, which appears to account for about 85 percent of all the matter in the universe. It doesn’t produce any energy of its own, and it almost never interacts with “normal” matter. But astronomers “see” its presence through its gravitational pull on the visible matter around it.

For a long time, the best explanation seemed to be some type of especially heavy particle. But experiments that look for it have come up empty.

So scientists have devised other possible explanations. One is an especially lightweight particle – as little as a billion billionth of the mass of the lightest bit of normal matter. Another is primordial black holes – tiny but heavy objects created in the Big Bang. And one of the wildest explanations is “tachyons” – hypothetical particles that would move faster than light.

Some scientists argue that dark matter doesn’t exist at all. One team, for example, suggests the universe is much older than the currently accepted 13.8 billion years. If so, then dark matter wouldn’t be needed – and neither would the game of hide-and-seek.

More about dark matter tomorrow.

Script by Damond Benningfield