We report on preliminary results from the Fermi High-Latitude Extended Sources Catalog (FHES), a comprehensive search for spatially extended gamma-ray sources at high Galactic latitudes ($|b|>5^circ$) based on data from the Fermi Large Area Telescope (LAT). While the majority of high-latitude LAT sources are extragalactic blazars that appear point-like within the LAT angular resolution, there are several physics scenarios that predict the existence of populations of spatially extended sources. If Dark Matter consists of Weakly Interacting Massive Particles, the annihilation or decay of these particles in subhalos of the Milky Way would appear as a population of unassociated gamma-ray sources with finite angular extent. Gamma-ray emission from blazars could also be extended (so-called pair halos) due to the deflection of electron-positron pairs in the intergalactic magnetic field (IGMF). The pairs are produced in the absorption of gamma rays in the intergalactic medium and subsequently up-scatter photons of background radiation fields to gamma-ray energies. Measurement of pair halos could provide constraints on the strength and coherence length scale of the IGMF. In a dedicated search, we find 21 extended sources and 16 sources not previously characterized as extended. Limits on the flux of the extended source components are used to derive constraints on the strength of the IGMF using spectral and spatial templates derived from Monte Carlo simulations of electromagnetic cascades. This allows us to constrain the IGMF to be stronger than $3times10^{-16},$G for a coherence length $lambda gtrsim 10,$kpc.
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