The night sky is full of stars, but they are only a small part of everything that exists in the universe. Astronomers have long noticed a strange thing: galaxies rotate so fast that they should fly apart, yet they don't. Something invisible holds them together, like invisible threads linking beads on a spinning wheel.
This "something" was named dark matter. It doesn't shine, doesn't reflect light, but it attracts ordinary matter. Without it, galaxies couldn't have gathered into the clusters we see. Dark matter also explains how light from distant galaxies is bent, as if you were looking through warped glass.
But there is another path: maybe we don't need new matter, but a change to the law of gravity. Some scientists propose tweaking Newton's formula to do away with invisible matter. So far, no alternative explains everything, so the search continues.
If you took a sugar cube of dark matter, it would weigh as much as a mountain, yet be completely invisible.
In the TV series "Dark Matter," the characters wake up on a ship with no memory — much like scientists who still don't know what this invisible substance is made of.