Personally only two:
- Pharmaceutical companies holding back better medicine and treatment options because they're still making billions off of their current ones
- The Governments (especially the USA) knowing more about the existance of extraterrestrial life than the public does
Most Helpful Guy
Yes, number one sounds a least very plausible. I somewhat believe in that one too. As for your second one, I am practically certain that this is wrong and really only a conspiracy theory (though it would be nice if it were true :-)). The reason I can be so sure about it is because I have always been extremely interested in astronomy and I know quite a bit about it. If you read enough about the universe, you realize that the numbers we are dealing with here are practically unimaginable. Because the universe is so incredibly huge, it is very likely that somewhere out there, there might be another planet with life on it. However, for the exact same reason (the universe being sooooo huge), it's also practically impossible for aliens to travel to us (let alone find us!). Even our by far closest neighbor-solar-system, Alpha Centauri, is 4,5 light years away. That means: even if there were aliens living in Alpha Centauri (which we know they don't because none of the planets there are in an orbit in the so-called Goldilock zone where life is possible) and even if this aliens were able to build a space ship that can fly with the speed of light (which we know is physically impossible), it would take them 4,5 years to get there. And this is our closest neighbor! The next galaxy outside of the milky way is already tens of thousands of light years away! Physicists have calculated that flying faster than 10% of the speed of light is technically impossible because you'd have to use way more energy that even exists on planet earth. And then there are other problems like mass and stuff like that (theory of relativity says that the speed increases the mass). So, even if there were some aliens in our neighbor galaxy who were so smart they could build a super awesome space ship that flies with dark energy or something like that and gets to 10% of the speed of light, it would take them hundreds of thousands of years to get here. Rather unlikely, don't you think?
However, one other conspiracy theory that I strongly believe in is that the Bush clan knew about 9/11 before it happened and didn't do anything about it. There's also the conspiracy theory that the Bush clan or the CIA are actually responsible for it. I'm not quite sure about that one. A friend of my parents is architect and he keeps insisting that it's statically impossible for a skyscraper to fall down to nicely after a plane hits it. I'm not so sure about that but I definitely think the US government knew about it.2