The Andromeda galaxy is the closest spiral galaxy to us. It harbors a massive dark matter halo which may span up to ∼600 kpc across and comprises ∼90% of the galaxy’s total mass. This halo size translates into a large diameter of 42 degrees on the sky for an M31–Milky Way distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with gamma-ray telescopes. Using data from the Fermi Large Area Telescope, we make a detailed study of the gamma-ray emission between 1-100 GeV towards Andromeda's outer halo, and perform an in-depth analysis of the systematic uncertainties  related to the observations. In this talk, I will discuss these results and implications for dark matter.