Danger from outer space
When thinking about biggest existential threats to our planet, excluding ourselves nuking the planet, or spreading some devastating biological weapon, there are numerous external threats, lurking on us from outer space.
Apart from asteriods, nearby supernova explosions, powerful solar flares directed to Earth, nearby black holes sucking in the whole Solar system, there is something even more frightening, Gamma-ray bursts a.k.a. GRBs. Why I'm saying this is because they can wipe us out without us even being aware of disaster coming.
Cosmic snipers
These cosmic snipers are so energetic that we can detect them even if they originate billions of light years away from our home, releasing more energy in one second than the sun will in it's entire lifetime. They can last anywhere from a few milliseconds to several minutes, depending on how they where created, by supernova implosion, or buy merger of two neutron stars colliding into each other.
GRBs are not rare!
Universe is full of GRBs, there is so many of them that here on Earth we detect on average one GRB per day, but luckily it originates too far away from us to pose and real threat to our existence.
They are shooting randomly all around the Universe, and one could eventually hit Earth one day, if it was sufficiently close, and destroy life as we know it. If a GRB was pointed in our direction, it could wipe us out even from a distance of a few thousand light years away. It really deserves to be called cosmic sniper.
Where are all the aliens?
GRBs could also be the reason why we haven't yet seen intergalactic empires, simply because they could be erasing civilizations like ours all the time.
In fact, GRB that will destroy us is probably already on its way to our intersection point and the encounter can happen now, tomorrow, or in a million years, nobody really knows, since it travels at speed of light, we will know when it hit us, but it will already be too late.
Enjoy the rest of the day :D