We report on a search for electron antineutrinos ( ) from astrophysical sources in the neutrino energy range 8.3-30.8 MeV with the KamLAND detector. In an exposure of 6.72 kton-year of the liquid scintillator, we observe 18 candidate events via the inverse beta decay reaction. Although there is a large background uncertainty from neutral current atmospheric neutrino interactions, we find no significant excess over background model predictions. Assuming several supernova relic neutrino spectra, we give upper flux limits of 60-110 cm-2 s-1 (90% confidence level, CL) in the analysis range and present a model-independent flux. We also set limits on the annihilation rates for light dark matter pairs to neutrino pairs. These data improve on the upper probability limit of 8B solar neutrinos converting into , (90% CL) assuming an undistorted shape. This corresponds to a solar flux of 60 cm-2 s-1 (90% CL) in the analysis energy range.