  Line emission from dark matter is well motivated for some candidates e.g.
sterile neutrinos. We present the first search for dark matter line emission in
the 3-80keV range in a pointed observation of the Bullet Cluster with NuSTAR.
We do not detect any significant line emission and instead we derive upper
limits (95% CL) on the flux, and interpret these constraints in the context of
sterile neutrinos and more generic dark matter candidates. NuSTAR does not have
the sensitivity to constrain the recently claimed line detection at 3.5keV, but
improves on the constraints for energies of 10-25keV.
