The “halo” that surrounds the Milky Way Galaxy is dark but heavy. It’s much more massive than the galaxy’s bright disk, but we don’t see much there. So the halo must be filled with dark matter. It produces no detectable energy, but it reveals its presence through its gravitational pull on the matter we can see.

The leading idea says dark matter consists of some type of exotic particle. But efforts to find such particles have come up empty.

Astronomers have also looked to see if the dark matter might consist of MACHOs – massive compact halo objects. The list of candidates includes faint stars, dead stars, black holes, free-floating planets, and brown dwarfs.

Such objects are extremely faint. But they can sometimes brighten – not directly, but by magnifying the light of stars behind them.

The technique is known as gravitational lensing. When one massive body passes in front of another, it causes the background object to get much brighter. The flare-up can last from hours to months. How long it lasts, and how much the background star brightens, reveals details about the lensing object. And that reveals the type of object.

Searches for gravitational lenses have found many planets, faint stars, and even the first “rogue” black hole – one that couldn’t be seen any other way. But there just aren’t enough MACHOs to account for more than a small fraction of the dark matter – leaving us in the dark about its nature.

Script by Damond Benningfield