Quantum mechanics

Substructure

Scale (ratio)

Weak gravitational lensing

Engineering

Cosmology

Redshift

Gravitation

Galaxy

Cold dark matter

Astrophysics

Scalar field dark matter

Computer science

Galaxy cluster

Gravitational lens

Dark energy

Physics

Cluster (spacecraft)

Astronomy

Structural engineering

Programming language

Hot dark matter

Dark matter

Science

Cold dark matter (CDM) constitutes most of the matter in the Universe. The interplay between dark and luminous matter in dense cosmic environments like galaxy clusters is studied theoretically using cosmological simulations. Observed gravitational lensing is used to test and characterize the properties of substructures - the small-scale distribution of dark matter - in clusters. An apt metric, the probability of strong lensing events produced by dark matter substructure, is devised and computed for 11 galaxy clusters. We report that observed cluster substructures are more efficient lenses than predicted by CDM simulations, by more than an order of magnitude. We suggest that hitherto undiagnosed systematic issues with simulations or incorrect assumptions about the properties of dark matter could explain our results.

An excess of small-scale gravitational lenses observed in galaxy clusters