a) Wandering black holes are big balls of God knows what, floating around the universe sucking away anything and everything in their path, and if one was to wander close enough to our solar system, it could spell the end of existence for mankind and Earth as we know it.
b) The Large Hadron Super Collider in Switzerland has the ability to accelerate protons towards each other at 99.999999% the speed of light, and can recreate conditions similar to those of the Big Bang. The one slight downside to this is that the Collider may mistakenly create a massive black hole which destroys Earth, along with the rest of our solar system. Oopsy daisy!
The 1300s saw the Black Plague claim the lives of between 75 and 200 million people. The 15th and 18th centuries witnessed smallpox kill around the same. Clearly man is no stranger to global pandemics, and the chances of another happening at some point are surprisingly high. Couple this with the fact that human beings are now more mobile than ever, and the speed at which a fatal new pandemic can spread is pretty terrifying.
Ever heard of technological singularity? The theory goes that at some point artificial intelligence will become so powerful that it not only exceeds human intelligence, but actually becomes capable of expanding its own intelligence without our help. In the eyes of the then superior AI, human beings will be seen as redundant, and pretty soon afterwards we'll be seen as a problem. Now I don't know what the robot overlord solution for a human "problem" will be, but I imagine it's nothing good.
Alien Invasion, or a New Superior Species
a) If the Drake Equation is correct, it is entirely possible that intelligent alien life exists somewhere in the universe. And if popular science fiction is correct, there is every chance that this alien life means us harm. Many theorists believe that human beings could be subject to farming/enslavement by a superior alien race were they to invade, just like we already do with most other members of the animal kingdom.
b) Rather than from space, another more "grounded" theory proposes that through the evolution process a superior species may one day be created on Earth, repositioning man's place on the food chain to second (and potentially first on the dinner menu).
Did you know that there are six known supervolcanoes around the world, just waiting to erupt? Though the last known eruption of one was around 70,000 years ago, any one of these six could theoretically send the whole world into a grey, post-apocalyptic, soot-and-ash-filled volcanic winter.
It's chilling how easy it would be for one scientist with some spare money and a broken moral compass to design and engineer a deadly virus, which could wipe out the populations of entire countries is no time at all... just think on that.
The Sun Expanding
The scariest thing about this one is that it's a 100% certainty. One day, many years from now, the sun will have grown enough in age and size that it will become a red giant, and will engulf our entire solar system.
Russia, North Korea, ISIS- the threats to global stability are very real. All it takes is for one world leader to have a bad day, or a nuclear warhead to fall into the wrong hands, and suddenly a deadly global war has been waged...
Considering that the world's human population has doubled to 7.1 billion in the last 45 years alone, it is scarily likely that we are going to hit a critical mass in the very near future, with food and water soon to become hot commodities. You think your morning tube commute is hell? You haven't seen anything yet.