Dark matter does not emit, absorb, or reflect light. Its presence, if it truly exists, is known only through its gravitational pull on visible matter in space. This mysterious substance is believed to be the invisible scaffolding of our Universe, forming long filamentary structures -- the cosmic web -- along which galaxies form. Astronomers have discovered that there may be a missing ingredient in our cosmic recipe of how dark matter behaves. They have uncovered a discrepancy between the theoretical models of how dark matter should be distributed in galaxy clusters, and observations of dark matter's grip on those clusters. One way astronomers can detect dark matter is by measuring how its gravity distorts space through an effect called “gravitational lensing.” Researchers have found that small-scale concentrations of dark matter in clusters produce gravitational lensing effects that are ten times stronger than expected. This evidence is based on unprecedented detailed observations of several massive galaxy clusters by NASA's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope (VLT) in Chile.