The Great Filter
Our solar system is middle-aged, which is an important factor in this discussion because it means that there should be many, many solar systems with Earth-like planets out there that are much older than ours. Many of them could be as much as a billion years older, in fact, implying a colossal head start in the process of evolving a technologically complex civilization. With a billion extra years to play with, you would think they could've harnessed those galactic levels of energy to facilitate interstellar travel. So, again, where are they?
One of the best-known explanations for the notable absence of aliens in these parts is something known as the "great filter." The thought is that there might be some inevitable phenomenon baked into the cosmos that prevents life forms from developing to the point where they can communicate or travel across interstellar distances.
The question then would be, where in the timeline of evolution does the great filter occur? More specifically, and selfishly, where are we in relation to this alleged filter? That depends entirely on the mysterious nature of the filter. If, for instance, the filter happens to be a kind of auto-destruct feature that dictates that civilizations destroy themselves before they reach the necessary point of technological development, then we're doomed.
Taking a rosier view, the filter could occur earlier on the timeline. For the first billion years of its existence life was content to take the form of super-simple prokaryote cells. Perhaps the leap to complex eukaryote cells is the great filter. This would mean that while there might be tons of life out there, it's all just a bunch of uncommunicative prokaryotes [source: Foley].
Alternatively, the filter might be something like gamma-ray bursts, gigantic electromagnetic explosions that might periodically wipe out life forms before they grow into anything interesting. According to this theory, we're fortunate enough to be in period of relative astronomical stability allowing for long-term evolution. And if that's the case, we might be one of many species co-evolving at about the same pace, and we'll all burst onto the interstellar scene around the same time (in roughly 200 years) [source: MIT Technology Review].
Turning gloomy again, it could also be next to impossible for life to occur, in which case we're just an outrageous fluke, a blue-green miracle lost in a vast, otherwise lifeless universe. That, of course, would mean that we're utterly and completely alone and always will be.