Annihilation lines are [spectral lines](http://en.wikipedia.org/wiki/Spectral_line) caused by the collision of particle-antiparticle pairs. In the case of $e^-e^+$, the emission is at 511 keV. 

![enter image description here][1]  
([source](http://arxiv.org/abs/arXiv:1307.4198) (arXiv link))

However, because it is caused by a collision of particles, rather than an absorption-emission of a photon, the peak is [Doppler broadened](http://en.wikipedia.org/wiki/Doppler_broadening). This means that the peak is spread out over a few keV, rather than a very sharp feature only at 511 keV, as an emission spectra would be. This effect is true for all annihilation lines, not just $e^-e^+$.

The galactic plane is a hotspot for the 511 keV line, but we're not really sure (as far as I know, at least) what is the source for the positrons.

![enter image description here][2]  
([source](http://isdc.unige.ch/integral/gallery.cgi?SCIENCE))

Some heavy elements can produce positrons in supernova nucleosynthesis (e.g., the [${}^{56}{\rm Ni}$ chain](http://physics.stackexchange.com/questions/114880/no-tremendous-neutrino-flux-for-snia/114901#114901)). It's also possible that [low-mass X-ray binaries](http://en.wikipedia.org/wiki/X-ray_binary#Low-mass_X-ray_binary) can produce positrons via pair-production in outflows. Another possible source could be [dark matter annihilation](http://en.wikipedia.org/wiki/Dark_matter#Indirect_detection_experiments). As far as I know, no *single* source has declared as "the" source for galactic positrons.


  [1]: https://i.sstatic.net/qaQH6.png
  [2]: https://i.sstatic.net/JQedp.png