Joel Bregman applies his expertise in X-ray observation and theory to account for the “missing baryons” (normal matter) in galaxies. Based on the accepted ratio of normal to dark matter, galaxies seem to be missing some 70-95% of their baryonic material. Bregman and his students have been working to determine whether the missing material might reside in the halos of hot gas that surround galaxies, as many astronomers have proposed. Using a variety of techniques to measure these hard-to-detect halos, they’ve demonstrated that the missing baryons cannot be accounted for in the hot gas halo, for example, around the Milky Way.