Cloudscape

Self-replicating spacecraft could replicate until there were many enough to build even greater self-replicating spacecraft, which could then in turn replicate to build even greater spacecraft, so that through self-replicating spacecraft one could eventually create megastructures which could manipulate the solar system in any way theoretically possible. In this, these self-replicating spacecraft would be similar to self-replicating nanomachines, which could build even smaller self-replicating nanomachines and so on, so that nanomachines could eventually manipulate molecules in any theoretically possible way.

In an infinite, or even just a very large Universe, there must have been at least some race which had lived long enough to achieve a state of godlike power and enlightenment. It is very possible that most sentient species would not have come that far, but even so, part of them would have, and in an infinite and eternal Universe, it would not matter how few there would be. Even in a very large and very old universe, it would still not matter very much how few there would be. Considering that as many of sixty have been observed as of 20/12/2008 which appear to be in the habitable zone of their star, and considering that there must be many, many more which we have as yet failed to discover with our currently primitive means of observing exoplanets, it is nothing less than prejudice to claim that there is nothing "out there."
Given the pains we take to attempt to communicate with extraterrestrial intelligence, if there are indeed intelligences besides our own, at least some of them would have taken the initiative to send forth Von Neumann probes. These are hypothetical spacecraft which would travel from exoplanet to exoplanet to explore, sending back information about their explorations either waiting for instructions or, more likely, make decisions on their own accord which would, with sufficient AI, get most obstacles out of the way. Every time they would have reached another exoplanet, they would replicate, meaning that they would produce more of their own kind, much like robots in factories today are already produced by other robots. Upon their replication, their now greater numbers could explore yet other exoplanets. Supposing these probes were nanoscale or even microscale, their mass would be so low that near-light speed would not require much energy -- - this would merely require more advanced miniaturization than we have today.
This energy could be provided mostly from an external source when the probe is still on or near its last exoplanet, for instance by the other probes, which then renew their own energy before departing themselves. Once it would be in outer space, its speed would not be braked by an atmosphere and could theoretically go on for many light years if it survived - it could use smaller amounts of energy to slightly adjust its course, to avoid obstacles or to enter into orbit around its target exoplanet. Because of their low mass, they could also be much, much greater in number, so that the chance that some would survive would be greater. In addition, they would not require much matter to replicate, as a proper micrometeorite would be enough. The most important problem with smaller sizes would be that less information could be contained in it. If the probe contained enough computing power, it could learn to communicate with us; this AI could be contained within a single computer, but it could also be contained within countless nanorobots. In addition, if it contained enough information, the computer could improve our knowledge by itself, for instance, or even improve ourselves, our very DNA and our thoughts, so as to "uplift" us as it is called in transhumanist jargon. The question here, of course, is whether we would want this, but some people among us certainly would (transhumanists, extropianists, many Buddhists, many mystics).
We have ourselves nearly achieved such level of technological sophistication, being at least a few decades and at most a few centuries away from it, supposing that our further development is unhindered by extinction level events (the most likely of which would probably be world-threatening terrorism). Since there occurred no actual events whatsoever which threatened to wipe out the entire human race since it arose, we may assume that many sentient species would probably have achieved the technology needed to build nanoscale Von Neumann probes. These could have explored the Universe practically at near-light speed, with only relatively very slight delays to replicate every time they would have reached new exoplanets.
Assuming that there have been such civilizations in the galaxy (four hundred billion stars), then they would have found us by now if they had existed at least fifty thousand years ago, which is only about fifteen thousand years before the first traces of the Cro-Magnon man showed up. Sensing intelligent life, their probes could have given at the very least some information about their species and their position in the universe. Given another hundred thousand years, their species themselves could have interfered by controlling the probes directly, so that even if their AI was too limited to make them communicate with us, the species themselves would have done so - the question is then if by that time this species would still exist or have interest in communicating with us. This would perhaps not have been necessary, however, as it is likely that if such probes had been sent at all, their AI could perhaps have been strong enough to communicate with us by itself, albeit in some primitive way.
Even supposing that we have been alone in the galaxy, there must have been some civilization whose Von Neumann probes could have reached us from somewhere millions of light years away. There is no reason why the probes would ever stop in their mission to find life unless the species which had initiated it would also itself terminate it. Our local group contains 35 galaxies and is about 10 million light years across - it would therefore have taken at most five million years for any sufficiently advanced species in the local group to reach us, and there are many trillions of stars in the local group. On biological time scale, five million years is just a minute. Five million years ago, the first hominids started to arise, so it seems reasonable to assume that at this time there had been plenty of races elsewhere in the local group which had become more advanced than we are today; the same can still more or less be assumed for hundreds of millions of years ago - at this point whatever intelligence there was on earth was still unsophisticated, but there are plenty of planets, including, probably, habitable planets, which arose hundreds of millions or even billions of years earlier.
Thus again, the question arises: where is everyone?