There is a 50% chance that a GRB took place during the last 500 Myr causing one of the major mass extinction events. The probability of a lethal GRB is much larger in the inner Milky Way, making it inhospitable to life. The safest environments for life are the lowest density regions in the outskirts of large galaxies and life can exist in only ~10% of galaxies.
the most plausible explanation of the fermi paradox yet.
Gregor J. Rothfuss 