  The signature of positron annihilation, namely the 511 keV $\gamma$-ray line,
was first detected coming from the direction of the Galactic center in the
1970's, but the source of Galactic positrons still remains a puzzle. The
measured flux of the annihilation corresponds to an intense steady source of
positron production, with an annihilation rate on the order of
$\sim10^{43}$~e$^{+}$/s. The 511 keV emission is the strongest persistent
Galactic $\gamma$-ray line signal and it shows a concentration towards the
Galactic center region. An additional low-surface brightness component is
aligned with the Galactic disk; however, the morphology of the latter is not
well constrained. The Compton Spectrometer and Imager (COSI) is a balloon-borne
soft $\gamma$-ray (0.2--5 MeV) telescope designed to perform wide-field imaging
and high-resolution spectroscopy. One of its major goals is to further our
understanding of Galactic positrons. COSI had a 46-day balloon flight in
May--July 2016 from Wanaka, New Zealand, and here we report on the detection
and spectral and spatial analyses of the 511 keV emission from those
observations. To isolate the Galactic positron annihilation emission from
instrumental background, we have developed a technique to separate celestial
signals utilizing the COMPTEL Data Space. With this method, we find a
7.2$\sigma$ detection of the 511 keV line. We find that the spatial
distribution is not consistent with a single point source, and it appears to be
broader than what has been previously reported.
