When searching for dark matter, astronomers must go on a sort of “ghost hunt.” That’s because dark matter is an invisible substance that cannot be seen directly. Yet it makes up the bulk of the universe’s mass and forms the scaffolding upon which galaxies are built. Dark matter is the gravitational “glue” that holds galaxies as well as galaxy clusters together. Astronomers can detect its presence indirectly by measuring how its gravity affects stars and galaxies.

The mysterious substance is not composed of the same stuff that makes up stars, planets, and people. That material is normal “baryonic” matter, consisting of electrons, protons, and neutrons. However, dark matter might be some sort of unknown subatomic particle that interacts weakly with normal matter.

A popular theory holds that dark matter particles don’t move very fast, which makes it easier for them to clump together. According to this idea, the universe contains a broad range of dark matter concentrations, from small to large.

Astronomers have detected dark matter clumps around large- and medium-sized galaxies. Now, using Hubble and a new observing technique, astronomers have found that dark matter forms much smaller clumps than previously known.

The researchers searched for small concentrations of dark matter in the Hubble data by measuring how the light from faraway quasars is affected as it travels through space. Quasars are the bright black-hole-powered cores of very distant galaxies. The Hubble images show that the light from these quasars images is warped and magnified by the gravity of massive foreground galaxies in an effect called gravitational lensing. Astronomers used this lensing effect to detect the small dark matter clumps. The clumps are located along the telescope’s line of sight to the quasars, as well as in and around the foreground lensing galaxies.