First, we evolve a DG stream in a time-dependent Milky Way–LMC interaction described by basis function expansions (BFEs). We find that the stream is significantly perturbed by the deformations, predominantly by the Milky Way dipole. Then, we develop an information theory approach to find the most informative streams on the time-dependent Milky Way dipole. These streams are long and wide, with a large apocentre. The constraints for the perturbation are ∼ 1 magnitude worse than for halo parameters. Then, we look into the bias of stream fits by fitting a stream evolved in the Milky Way–LMC simulation with current state-of-the-art, rigid techniques. We find an underestimated Milky Way mass, an overestimated LMC mass, and stronger flattening in the fits.

To make use of this opportunity, we need detectors in this ‘collider’. Some of the most sensitive tracers to the gravitational potential of the Galaxy are stellar streams. Streams are disrupted globular clusters and dwarf galaxies (DGs), forming long, filament-like structures in the halo. In this thesis, we explore whether stellar streams are affected by the deforming Milky Way and LMC, which streams are particularly informative on these deformations, and how omitting these deformations biases stream fits.

The nature of dark matter is one of the biggest open questions in modern-day astrophysics. Recent studies have shown that the dark matter halo of the Milky Way is deforming due to the infalling Large Magellanic Cloud (LMC). These two haloes serve as a dark matter ‘collider’. This dark matter ‘collider’ provides a unique testbed for the nature of dark matter.

Finally, I will end with an overview of future directions for this field. Our results reveal the need for a time-dependent Milky Way model. To fit the Milky Way streams with these time-dependent models, we need to utilise the functionality of BFEs. This will enable us to probe how dark matter haloes deform and test different dark matter theories.