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Only in the Milky Way is it possible to conduct an experiment which usesstellar streams to detect low-mass dark matter subhaloes. In smooth and statichost potentials, tidal tails of disrupting satellites appear highly symmetric.However, perturbations from dark subhaloes, as well as from GMCs and the MilkyWay bar, can induce density fluctuations that destroy this symmetry. Motivatedby the recent release of unprecedentedly deep and wide imaging data around thePal~5 stellar stream, we develop a new probabilistic, adaptive andnon-parametric technique which allows us to bring the cluster's tidal tailsinto clear focus. Strikingly, we uncover a stream whose density exhibitsvisible changes on a variety of angular scales. We detect significant bumps anddips, both narrow and broad: two peaks on either side of the progenitor, eachonly a fraction of a degree across, and two gaps, $\sim2^{\circ}$ and$\sim9^{\circ}$ wide, the latter accompanied by a gargantuan lump of debris.This largest density feature results in a pronounced inter-tail asymmetry whichcannot be made consistent with an unperturbed stream according to a suite ofsimulations we have produced. We conjecture that the sharp peaks around Pal 5are epicyclic overdensities, while the two dips are consistent with impacts bysubhaloes. Assuming an age of 3.4 Gyr for Pal 5, these two gaps wouldcorrespond to the characteristic size of gaps created by subhaloes in the massrange of $10^6-10^7 M_\odot$ and $10^7-10^8 M_\odot$ respectively. In additionto dark substructure, we find that the bar of the Milky Way can plausiblyproduce the asymmetric density seen in Pal 5 and that GMCs could cause thesmaller gap.

A sharper view of Pal 5's tails: Discovery of stream perturbations with a novel non-parametric technique