Dark matter is believed to make up to 85 per cent of the total mass of the Universe; however, most of its properties have yet to be constrained. At the present time, the most tested and the most studied cosmological model for the formation of structures in the Universe is the Cold Dark Matter (CDM) paradigm, where the dark matter is assumed to be cold and collision-less. This model has been very successful in describing the Universe on large scales and in reproducing numerous observational results. Due to the challenging nature of the observations in this regime, however, the CDM paradigm is still largely untested at the smallest galactic and sub-galactic scales, where definite theoretical predictions from different dark matter models mostly differ. All dark matter models predict that structures form via gravitational instability from primordial density fluctuations in the early Universe and grow hierarchically in time through the merging and accretion of smaller objects. As a result, it is expected that the dark matter distribution should be clumpy and that there should be many low-mass dark structures scattered around other more massive galaxies. The specific scale at which structure is expected to be clumpy strongly depends on the assumed physics of the dark matter particles. For example, the CDM model predicts that the Universe should be clumpy all the way down to the smallest sub-solar scales and that a rich population of low-mass structures should exist.
On the other hand, models that assume a warm dark matter (WDM) particle predict the existence of much fewer low-mass structures and the presence of a cut-off mass below which the dark matter distribution should be smooth. Measuring the abundance of small-mass dark structures is, therefore, a key constraint to the nature of dark matter and structure formation physics. Unfortunately, most of these small-mass structures are expected to be completely dark or extremely faint and not directly observable. These small objects can, therefore, only be observed and quantified by using strong gravitational lensing.
The primary aim of this ERC project is to use strong gravitational lensing to constrain the nature of dark matter observationally. To this end, we developed an advanced lens modelling code that can efficiently and robustly analyse large interferometric data directly in visibility space. This novel technique can be used to probe the dark matter distribution on sub-galactic scales within the lens galaxies and along their line of sight. By combing our lens modelling code with high-resolution cm-observations taken with the Global VLBI array we derived one of the tightest constraints on the particle mass of Fuzzy dark matter to date, and probed, for the first time, the subhalo and halo mass functions below 10^7 Msun. From the analysis of sub-mm observations taken with ALMA, we demonstrated that lens galaxies are not simple elliptical and smooth objects as typically assumed. Instead, they are characterised by complex mass density distributions with radial and angular structures. These features need to be taken into account for a robust inference on the nature of dark matter.
