  In this letter, we introduce a new method of image stacking to directly study
the undetected but possible gamma-ray point sources. Applying the method to the
Australia Telescope 20 GHz Survey (AT20G) sources which have not been detected
by LAT on Fermi, we find that the sources contribute (10.5+/-1.1)% and
(4.3+/-0.9)% of the extragalactic gamma-ray background (EGB) and have a very
soft spectrum with the photon indexes of 3.09+/-0.23 and 2.61+/-0.26, in the
1-3 and 3-300GeV energy ranges. In the 0.1-1GeV range, they probably contribute
more large faction to the EGB, but it is not quite sure. It maybe not
appropriate to assume that the undetected sources have the similar property to
the detected sources.
