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The Galactic center excess (GCE) remains one of the most intriguingdiscoveries from the Fermi Large Area Telescope (LAT) observations. We revisitthe characteristics of the GCE by first producing a new set of high-resolutiongalactic diffuse gamma-ray emission templates, which are ultimately due tocosmic-ray interactions with the interstellar medium. Using multi-messengerobservations we constrain the properties of the galactic diffuse emission. Thebroad properties of the GCE that we find in this work are qualitativelyunchanged despite the introduction of this new set of templates, though itsquantitative features appear mildly different than those obtained in previousanalyses. In particular, we find a high-energy tail at higher significance thanpreviously reported. This tail is very prominent in the northern hemisphere,and less so in the southern hemisphere. This strongly affects one prominentinterpretation of the excess: known millisecond pulsars are incapable ofproducing this high-energy emission, even in the relatively softer southernhemisphere, and are therefore disfavored as the sole explanation of the GCE.The annihilation of dark matter particles of mass $40^{+10}_{-7}$ GeV (95$\%$CL) to $b$ quarks with a cross-section of $\sigma v = 1.4^{+0.6}_{-0.3} \times10^{-26}$ cm$^{3}$s$^{-1}$ provides a good fit to the excess especially in therelatively cleaner southern sky. Dark matter of the same mass rangeannihilating to $b$ quarks or heavier dark matter particles annihilating toheavier Standard Model bosons can combine with millisecond pulsars to provide agood fit to the southern hemisphere emission. As part of this paper, we makepublicly available all of our templates and the data covariance matrix we havegenerated to account for systematic uncertainties.[abridged]

The Return of the Templates: Revisiting the Galactic Center Excess with Multi-Messenger Observations