  Recent observations indicate that a high production rate of positrons (strong
511 keV line) and a significant amount of excess GeV gamma-ray exist in our
Galactic bulge. The latter issue can be explained by $\sim 40$ GeV dark matter
annihilation through $b \bar{b}$ channel while the former one remains a
mystery. On the other hand, recent studies reveal that a large amount of high
density gas might exist near the Galactic Centre million years ago to account
for the young, massive stars extending from 0.04 pc - 7 pc. In this article, I
propose a new scenario and show that the 40 GeV dark matter annihilation model
can also explain the required positron production rate (511 keV line) in the
bulge due to the existence of the high density gas cloud near the supermassive
black hole long time ago.
