Consider the following: there are about 300 billion stars in our galaxy. We have every reason to believe that these stars typically support a wide variety of types of planets. In addition, there are hundreds of billions of galaxies, similar to our own.
Current estimates for stars in the visible universe are about 70 sextillion (7 × 10
22). Our solar system and Earth formed a relatively late 10 billion years after the universe began. Other stars had formed and died by then. Life on Earth formed a few hundred million years after the Earth formed.
Thus, it would seem there was both ample locations and ample time for life to have formed many many times other than on Earth. Our universe and galaxy should be teeming with life. With so much life, and with that life having a considerable head start on us, the unavoidable question is:
Why Haven't We Been Contacted By Aliens?
This question is known as the
Fermi Paradox, and if you've never read about it I suggest you check out the Wiki article. Here I will attempt to cover some of the major explanations for why there has been no contact with aliens.
As I said above there are at least 7 × 10
22 stars in the visible universe. To date we've discovered over 500 planets outside our solar system. However, those planets are mostly quite different from Earth, and unlikely to support life. This is largely a result of the conditions that make planets easier to detect (very large mass, very close and short orbit). We have only
just begun to search for Earth like planets, but we have already found a few that could qualify. We have no reason to think that Earth like planets are particularly rare. Even if they were as rare as 1% of star systems, the absurd number of stars would mean there would still be an unimaginable number of Earth like planets. I don't think that there is a lack of suitable locations.
With only one example of a planet that supports life, we have very little idea of what kind of conditions are needed for life to begin. There are a few things about Earth that are unusual, and if these things are necessary for life to begin it could mean that life just doesn't happen that often. These go beyond a planet being Earth like (which usually means small, rocky, and having liquid water). We have a large tilt giving us seasons, a large moon giving tides, and our star is located in a rather sparse area, protecting us from various cosmic catastrophes. You can read some more of the interesting things
Earth has going for it on Wiki.
In addition, while evolution does an excellent job of explaining how life can go from a basic replicating stage to the diverse complex array we are a part of today, we have no explanation for how life can
spontaneously begin.
Some experiments have shown amino acids can form from sterile conditions similar to what would have been present on the early Earth. However, while amino acids are an important component of life, they themselves are not life.
However, I don't think there is anything about the Earth that is that uncommon. Nor is there anything that seems like it would be totally insurmountable. Life on Earth began within a half a billion years of Earth forming. A typical star system would last about ten billion years. It seems like life began very shortly after it was possible on Earth. Given our one example, I see no reason to assume life is particularly unlikely given reasonable conditions.
Intelligent Life is Rare
Many people have a flawed impression about evolution. Specifically, that it leads to better and more advanced life. Better is too broad a word to have much meaning, and while it certainly can lead to more advanced life, it isn't a foregone conclusion.
Simply put, evolution is just a process where genes that cause themselves to be more likely to be passed on (by producing organisms better adapted to their environments, or more likely to successfully reproduce), tend to become more common. These genes represent small changes, which over very long periods of time can lead to great diversity. Note that neither 'better adapted to their environments' nor 'more likely to successfully reproduce', is equivalent to 'more advanced'. More advanced may be a way to achieve those goals, but in and of itself is not a goal.
The
evolution of human intelligence took 'only' 7 million years. Life had existed for about a billion years, and complex land based vertebrates had existed for several hundred million years. It seems like human level intelligence could have evolved earlier if it was a significant evolutionary advantage.
Another evolutionary misconception is that improvements in speed, size, strength, intelligence, etc are advantages. While these things could be an advantage, they are trade-offs. They come with an increase in calorie demands. If the improvement doesn't allow for a corresponding increase in calorie intake, then it is a disadvantage. The human brain is no exception to this; it
consumes about 20% of the calories we take in.
It is not hard to imagine how the increased intelligence of humans may not have been enough to offset the increased caloric demands that went along with it. Would a human, without the accumulation of knowledge passed down, be any better at surviving than another great ape? The fact that we are here would seem to be evidence that the increased intelligence would be an actual advantage. However, there could have been a unique set of selective pressures that caused intelligence to be beneficial. If this is the case, it could be that diverse complex life is common, but that intelligence is rare or unique.
Intelligent Life Tends to Destroy Itself
The ability of the human race to destroy itself is a unique and recent development. More than a hundred years ago it wouldn't have been possible for any group of humans to wipe out the entire human race. With nuclear weapons, this became possible. There are other methods that would have the same result as well, some that could be accidents. It could be argued that as intelligent life develops more and more technology the number of ways it could destroy itself only increases. Eventually it becomes certain that one of these things will happen.
That being said, I've never been a fan of this explanation. It would seem we are unlikely to wipe ourselves out anytime soon, and we have already reached a point where we are detectable from space. In a hundred more years it seems reasonable we could be taking the first steps into other solar systems. Once we reach that point, I would think we'd be a lot harder to wipe out. Again, with the number of stars it would only take a very small percentage of civilizations that didn't wipe themselves out to lead to a huge number of civilizations that should be present.
Faster Than Light Travel is Impossible
While simply accelerating to the speed of light is impossible there are a number of 'loop-holes' that would allow for travel between two points in less time than light would take. If it turns out that even these are impossible and that it is truly impossible to travel astronomical distances faster than light then this would pose a significant problem for galactic colonization. There could be a few dozen current civilizations in our galaxy that we'd never contact due to the distances.
That being said, it would take at least 100,000 years to cross our galaxy. If there were thousands or more advanced civilizations it seems probable that some would be traveling around at close to the speed of light, and that some would have come across the Earth by now.
These are just the main reasons I think are most likely to be relevant.
Wikipedia goes into much more depth. In addition there is something called the
Drake equation, which aims to estimate the current number of advanced civilizations given a set of variables. If I had to make a guess as to the reasons, I'd have to say a combination of rarity of intelligent life and faster than light travel being impossible.
