Why is the universe silent? Why can't we find any aliens? Is there nobody out there? SETI has searched the skies very thoroughly by now. We should have found somebody. Many theories have been proposed as answers to this question, which is called Fermi's Paradox. In this writeup I will try to give an overview of the various explanations for the great silence.
One possibility is that we, the human race, are the only sentient species that will ever evolve. That theory, however, is rather improbable, considering the size and life span of the universe. Unless of course, you believe that there is a god who created humanity, and the cosmos is our playground.
Another possibility, the most probable one, is that there are no other sentient life-forms around at the moment, but there will be other intelligent life in the future, or there has been in the past. We've had radio for only a few decades, while the universe's been around for billions of years. How big are the chances that two sentient beings would coexist at the same time close enough to each other, each possessing radio technology?
That of course implies that a species goes through a very short time-span during which it can contact others by radio. This can be due to two reasons: Either a sentient species transcends shortly after getting radio (see butterfly hypothesis in the above writeup). Or, for a bleaker theory, all sentient species either exterminate themselves shortly after, if not before, getting radio technology. Or, if they don't, they'll colonize every solar system in this galaxy within a few millennia (an eyeblink on galactic time-scale). This would leave no space for other sentient beings to develop - the whole galaxy would be so crowded with that one species. Think population explosion.
This way we would have to be the first: If any other sentient species had evolved first, Earth would be an alien colony, and humanity would never have had the opportunity to evolve into sentient beings.
There are many other less probable theories, some of which I will now describe. Though improbable, they cannot be discounted, for one of them might just turn out to be true.
There is the theory that we're using a completely wrong medium of communication. Perhaps we're the only species as unreasonable as to use radio. Maybe all the aliens are happily chatting away telepathically, and haven't even noticed us poor telepathy-impaired beings. Maybe they use highly focused laser beams for communication, or some physical concept we don't even know of yet.
It might also be that the alien lifeforms are well aware of us, but are hiding. Why would they do that? For one, they might have heard our radio and TV transmissions and decided that they don't want contact with such an irrational species. Maybe they're afraid of us. Considering the pictures of violence we continually shout out into the void, this theory might just be true.
Or possibly they don't want to interfere with our development. That might be because they know they'd harm us if they would interfere with our development at this stage. Because they adhere to the Prime Directive from Star Trek. Or perhaps they're a bunch of darwinists who want to see whether we manage to survive on ourselves before they make themselves known.
Either way they'd probably reveal themselves to us at some point. Perhaps when we secure world peace and government. Or when our first space probe reaches another solar system. Or perhaps they're only waiting for the abolishment of the death penalty.
I'd love to attribute all these theories, but unfortunately I have no idea whatsoever about who first proposed them. /msg me if you know.