We have a workable and testable hypothesis for the origin of the
extragalactic
-ray background measured by
EGRET, viz., that it is made up
primarily of unresolved blazars. The GLAST
-ray telescope, to be
flown in the near future, will be able to test this hypothesis in three
ways, i.e., (a) by potentially resolving out and detecting
thousands of
more sources, (b) by measuring the remaining background flux, and (c) by
determining the shape of the EGRB up to TeV energies. The many new
ground-based detectors now under construction will supplement
this information by discovering new extragalactic sources of
-rays of
energies above 50 GeV.
On the other hand, the mystery of the origin of the EGRB in the MeV energy range must await a better determination of this background by a future dedicated satellite detector.