Most of these other civilizations that still exist in the galaxy today are likely young, due to the probability that intelligent life is fairly likely to eradicate itself over long timescales. Even if the galaxy reached its civilizational peak more than 5 billion years ago, most of the civilizations that were around then have likely self-annihilated, the researchers found. This last bit is the most uncertain variable in the paper; how often do civilizations kill themselves? But it's also the most important in determining how widespread civilization is, the researchers found. Even an extraordinarily low chance of a given civilization wiping itself out in any given century — say, via nuclear holocaust or runaway climate change — would mean that the overwhelming majority of peak Milky Way civilizations are already gone